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38Which is true of the-messages posted on Cancer Chat？
AThey are available to all visitors.
BTheir writers get paid for them.
CThey are only about cancer treatments.
DTheir writers don't have to register.

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下面的短文后列出了7个句子，请根据短文的内容对每个句子做出判断：如果该句提供的是正确信息，请选择A；如果该句提供的是错误信息，请选择B；如果该句的信息文中没有提及，请选择C。
Radiocarbon Dating
Nowadays scientists can answer many questions about the past through a technique called radiocarbon (放射性碳), or carbon-14, dating. One key to understanding how and by something happened is to discover when it happened.
Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at he University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(中子) and six neutrons (质子) in its nucleus. Carbon-14, or C-14, is a radioactive, unstable form. of carbon that has two extra neutrons (原子核). It returns to a more stable form. of carbon through a process called decay (衰减). This process involves the loss of he extra neutrons and energy from the nucleus.
In Libby's radiocarbon dating technique, the weak radioactive emissions (放射) from his decay process are counted by instruments such as a radiation detector and counter. He decay rate is used to determine the proportion of C-14 atoms in the sample being dated.
Carbon-14 is produced in the Earth's atmosphere when nitrogen (氮)-14, or N-14,
interacts with cosmic rays (宇宙射线). Scientists believe since the Earth was formed, the mount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years.
All life on Earth is made of organic molecules (分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14
atoms to other carbon atoms in their tissues (组织). Once an organism (有机体) dies it tops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon toms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is.
16 Nowadays many scientists depend on radiocarbon for dating age-old objects
A Right
B Wrong
C Not mentioned

下面的短文有15处空白，请根据短文内容为每处空白确定一个最佳选项。
Middle Age: A Low Point for Most
People around the globe hit the height of their misery and depression in middle (51) , a new international study suggests. The finding by British and American researchers was based on an analysis of well-being among approximately 2 million people in 80 nations. With few exceptions, the observation appears to apply across the board, regardless (52) gender (性别), culture, geography, wealth, job history, education, and marriage or parental status.
"The scientific fact seems to be that happiness and positive mental health follow a giant 'U' (53) through life," said study author Andrew J. Oswald, a professor of economics at Warwick University in Warwickshire, England. "For the average person, it's high when you're 20, and then it slowly (54) and bottoms out in your 40s. But the good news is that your (55) health picks up again, and eventually gets back to the high levels of your youth."
The finding was (56) on the pooling of several different sources of happiness data, including: two multi-decade happiness/satisfaction surveys (first launched in the 1970s), involving about 500,000 American and Western European men
and women; four rounds of the 80-nation "World Values Survey" (57) between 1981 and 2004 in North America, Eastern and Western Europe, Asia, Africa, Australia, and Central and South America; and a 2004-2007 survey (58) nearly 1 million Britons.
The bottom-line: For most people throughout the world, the highest probability for (59) striking is around 44 years of age.
In the United States, however, some as-yet unexplained (60) differences were observed, with happiness among men dipping the most in their early 50s, whereas women hit their nadir (最低点) around the age of 40.
The researchers cautioned that cheerful people tend to live longer than unhappy
(61) - a fact that might have skewed (使偏斜) the overall finding. But they also suggested that evidence of a happiness (62) might simply reflect a midlife choice to give up long-held but no longer tenable (守得住的) aspirations (志向), followed by a senior's sense of gratitude for having successfully endured
(63) others did not.
"That said, some might find it helpful simply to understand the general (64) of mental health as they go through their own life," said Oswald. "It might
be useful for people to realize that if they are (65) in their 40s this is normal. It is not exceptional. And just knowing this might help."
51
A place
B age
C height
D degree

15 Ten years after the event, her death still remains a puzzle.
A mist B fog
C mystery D secret

13 These paintings are considered by many to be authentic.
A faithful B royal
C genuine D sincere