题目内容
测得一松木试件，其含水率为11%，此时其顺纹抗压强度为64.8MPa。 根据以上情况，请回答下列问题： (1)标准含水量状态下其抗压强度为多少 (2)当松木含水率分别为20%、30%、40%时的强度各为多少 (3)分析含水率对松木顺纹抗压强度的影响规律(该松木的纤维饱和点为30%，松木的A为0.05)。
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For mothers-to-be, doctors worldwide advise taking a folic acid supplement. That"s because pregnant women with a deficiency of this vitamin have an increased chance of giving birth to a baby with serious birth defects. Yet a new mouse study shows that folic acid supplementation can itself sometimes increase the risk of birth defects or even cause the death of embryos. Experts caution, however, that the unexpected rodent results are too preliminary to require an immediate change in medical practices until more is known about how the vitamin influences development. People typically obtain folic acid, from consuming leafy vegetables, but not everyone gets enough from their diet, particularly pregnant women. Evidence from randomized clinical trials has shown that babies born to women who double their recommended daily dose of folic acid are between 40% and 50% less likely to have birth defects of the spine, skull, and brain. As a result, the United States has fortified most of its grains with this vitamin since 1998, and a handful of other countries have followed suit. But just how the vitamin influences embryonic growth remains a mystery. So developmental biologist Lee Niswander of the University of Colorado, Denver, set out to investigate folic acid supplementation in mice genetically predisposed to giving birth to embryos with neural tube defects. She and her colleagues fed five such strains of mutant mice food containing either 2 or 10 mg of folic acid per kilogram, which created a range of blood levels of the vitamin equivalent to that in the U. S. population. To the surprise of the researchers, in three of the five strains, the extra folic acid seemed to worsen the severity of birth defects rather than remedy them. In one of the mutant lines, eating the higher folic acid diet long term increased the chance that young were born with neural tube defects from 20 % to 60 % . And for another strain, many of whose embryos don"t naturally survive until birth due to their genetic problems, eating the higher folic acid food significantly increased the percentage of lost embryos. Niswander says it is clear that folic acid is good for human fetuses, but the new study makes her wonder whether high levels of the vitamin may be harmful in some circumstances. Still, she stresses that more data are needed before any serious reconsideration of how much folic acid to recommend for impending mothers. Roy Pitkin, a retired researcher of University of California, Los Angeles, who specialized in pregnant women"s nutrition and chaired an Institute of Medicine panel that in 2000 reviewed folic acid"s health effects also cautions against a rush to judgment: "It would really be throwing the baby out with the bath water to say that because of this one mouse study, we are going to question the food fortification. " Which of the following is true of the research led by Lee Niswander
For mothers-to-be, doctors worldwide advise taking a folic acid supplement. That"s because pregnant women with a deficiency of this vitamin have an increased chance of giving birth to a baby with serious birth defects. Yet a new mouse study shows that folic acid supplementation can itself sometimes increase the risk of birth defects or even cause the death of embryos. Experts caution, however, that the unexpected rodent results are too preliminary to require an immediate change in medical practices until more is known about how the vitamin influences development. People typically obtain folic acid, from consuming leafy vegetables, but not everyone gets enough from their diet, particularly pregnant women. Evidence from randomized clinical trials has shown that babies born to women who double their recommended daily dose of folic acid are between 40% and 50% less likely to have birth defects of the spine, skull, and brain. As a result, the United States has fortified most of its grains with this vitamin since 1998, and a handful of other countries have followed suit. But just how the vitamin influences embryonic growth remains a mystery. So developmental biologist Lee Niswander of the University of Colorado, Denver, set out to investigate folic acid supplementation in mice genetically predisposed to giving birth to embryos with neural tube defects. She and her colleagues fed five such strains of mutant mice food containing either 2 or 10 mg of folic acid per kilogram, which created a range of blood levels of the vitamin equivalent to that in the U. S. population. To the surprise of the researchers, in three of the five strains, the extra folic acid seemed to worsen the severity of birth defects rather than remedy them. In one of the mutant lines, eating the higher folic acid diet long term increased the chance that young were born with neural tube defects from 20 % to 60 % . And for another strain, many of whose embryos don"t naturally survive until birth due to their genetic problems, eating the higher folic acid food significantly increased the percentage of lost embryos. Niswander says it is clear that folic acid is good for human fetuses, but the new study makes her wonder whether high levels of the vitamin may be harmful in some circumstances. Still, she stresses that more data are needed before any serious reconsideration of how much folic acid to recommend for impending mothers. Roy Pitkin, a retired researcher of University of California, Los Angeles, who specialized in pregnant women"s nutrition and chaired an Institute of Medicine panel that in 2000 reviewed folic acid"s health effects also cautions against a rush to judgment: "It would really be throwing the baby out with the bath water to say that because of this one mouse study, we are going to question the food fortification. " Why do doctors recommend folic acid supplements for pregnant women
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下列选项中，属于昆虫的可见光波范围的是 nm。
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