题目内容
Insects which eat grain and other stored food can be partially controlled by cooling the grain. But the damage insects cause can be further limited by making them even more susceptible to cold, says a Canadian researcher.
Insect pests die if they freeze, but many have the ability to become "supercool", remaining alive even when the temperature drops to - 10℃. However, Paul Fields of the Agriculture Canada Research Station in Winnipeg has discovered that the ability of these insects to supercool can be seriously affected by "ice-nucleating-active bacteria". When the bacteria are present, the insects freeze and die at a higher temperature than normal.
Ice-nucleating-active bacteria are found naturally on most plant leaves. They are also grown commercially for use in cloud seeding and in making artificial snow.
Fields added these bacteria to wheat which was stored at a temperature of - 10℃ and contained the rusty grain beetle. He found that when the bacteria accounted for only 10 parts per million of the grain, the beetles' supercooling point rose from - 11.3℃ to - 7.1℃, so more died. At concentrations of 100 ppm or 1000 ppm, the bacteria increased the supercooling point to as high as -6.3℃, which resulted in more than 75 percent of the beetles dying.
Fields also applied the bacteria to two other species of grain beetle, Sitophilus granarius and Oryzaephilus surinamensis, keeping them at - 10℃. He found that more of these beetles also died. First, though, he gradually acclimatized some of the insects to cold, to simulate the increase in tolerance to cold which they normally develop as winter approaches. Fields found that the insects that had been acclimatized were less affected by cold than the beetles that had not been acclimatized, but that more of them still died at - 10℃.
Fields says that the technique of using ice-nucleating-active bacteria leaves no noxious chemical residues and poses little risk to the people who apply the treatment. It is also effective against insects which have become resistant to pesticides.
Which of the following statements about ice-nucleating-active bacteria is true？
查看答案
搜索结果不匹配？点我反馈
更多问题
The seating at a press conference is not by accident. The regulars have marked seats in the first three or four rows, and beyond that the seating is on a first-come, first-served basis. In a way that no one talks about, this allows a press secretary to let you know about his or the President's displeasure. If a reporter who has been in the front row walks in one day and finds he is sitting five rows back, he knows what has happened.
This fixed seating allows the President to know who is sitting where. Johnson studied the charts, and Nixson always knew where the reporters who mattered, in his view, were seated. He knew where he could go if he needed to change the subject. The lack of follow-up to an answer has always been one of the flaws of the press conference format. The press corps has never done a good job on it. I tried to go into a press conference with five questions I would like to ask, and a backup list of five more. But I had to be ready to follow up someone else's question.
There are other weaknesses in press conferences, of course, among them the fact that ninety-nine percent of the questions are political. Such issues as genetic-engineering, overpopulation, the global economy do not often get raised. We have not figured out yet what our responsibility should be reporting these issues before they get to be such immense problems.
Ideally, a presidential news conference should be held every ten days to two weeks, live, in an unstructured seating. Television works best when it puts you there, in a situation where the camera has the least influence on the person. With our improvement in technology, we are coming to that point soon. The new minicams spit our broadcast-ready videotape on the spot.
According to the passage, at a presidential press conference of about 200 people, will be reporters.
SECTION C NEWS BROADCAST
Directions: In this section, you will hear several news items. Listen to them carefully and then answer the questions that follow.
听力原文： Doctors yesterday warned amorous British couples not to try for a millennium baby as hospitals could be overloaded, and they might even end up with a handicapped child. The warning was issued as surveys showed as many as one in 10 British couples are trying for a baby to be born on the magic stroke of midnight.
What's the doctors' attitude toward British couples planning to have millennium babies？
听力原文： Pakistani President Pervez Musharraf said yesterday there was no danger of the country going to war with neighboring India, but that Pakistani forces would be ready to repel any aggression.
"There is no danger of war," Musharraf told reporters in the Pakistani capital of Islamabad. "We should have confidence in ourselves. We are not sitting idle. We are prepared for everything. There should not be any misunderstanding."
Tensions were raised this week as the two accused each other of links to killings in the two countries. India says it suspects the two gunmen who killed 28 people at an Indian temple on Tuesday had links to Pakistan-based Islamic militant groups.
Pakistan has denied any involvement in the temple massacre, and police in Karachi said there were indications of India intelligence agents behind the murder of seven Christian charity workers in the city. But India rejected the charges yesterday.
According to Pakistani President, the chances of the two countries going to war were ______
听力原文： The governors of 45 states agreed Sunday to develop common measures for establishing high school graduation rates, a step they said will help achieve their larger goal of making high school rigorous enough to help prepare students for an increasingly competitive global economy.
The agreement aims to replace a hodgepodge of measuring systems with a uniform. standard that will allow parents, students, educators and politicians to compare state graduation rates. Virginia Gov. Mark R. Warner(D) , the outgoing chairman of the National Governors Association, said that with uniform. data on graduation rates and eventually dropout rates, states will have tools to help them track and target efforts to push all students to graduate from high school.
Education experts say a key predictor of whether students eventually will graduate from college is not race or economic circumstances but whether they completed a rigorous course of study in high school. Warner has used his year as NGA chairman to spearhead the initiative to raise awareness of the weaknesses of U. S. high schools and establish higher standards and more difficult curricula.
Governors reiterated Sunday their belief that global competitiveness has left U.S. students in a precarious position, with an economy that demands greater skills but a high school system still designed for the old economy.
What is the main topic of this news？
微信支付 
支付宝支付