18. In experiments in which certain kinds of bacteria were placed in a generous supply of nutrients, the populations of bacteria grew rapidly, and genetic mutations occurred at random in the populations. These experiments show that all genetic mutation is random.
Which one of the following, if true, enables the conclusion to be properly drawn?
(A) Either all genetic mutations are random or none are random.
(B) The bacteria tested in the experiments were of extremely common forms.
(C) If all genetic mutations in bacteria are random, then all genetic mutations in every other life form arc random also.
(D) The kind of environment in which genetic mutation takes place has no effect on the way genetic mutation occurs.
(E) The nutrients used were the same as those that nourish the bacteria in nature.
20. Calories consumed in excess of those with which the body needs to be provided to maintain its weight are normally stored as fat and the body gains weight. Alcoholic beverages are laden with calories. However, those people who regularly drink two or three alcoholic beverages a day and thereby exceed the caloric intake necessary to maintain their weight do not in general gain weight.
Which one of the following, if true, most helps to resolve the apparent discrepancy?
(A) Some people who regularly drink two or three alcoholic beverages a day avoid exceeding the caloric intake necessary to maintain their weight by decreasing caloric intake from other sources.
(B) Excess calories consumed by people who regularly drink two or three alcoholic beverages a day tend to be dissipated as heat.
(C) Some people who do not drink alcoholic beverages but who eat high-calorie foods do not gain weight.
(D) Many people who regularly drink more than three alcoholic beverages a day do not gain weight.
(E) Some people who take in fewer calories than are normally necessary to maintain their weight do not lose weight.
18、若将结论修改成：These experiments show that all this certain kinds of bacteria genetic mutation is random. 选项D就成立了。
20、A选项，不管这个人是否又吃了肥猪肉，他现在的脂肪摄入量已经是超标了。还应该增肥。因此，这个选项没有解决矛盾。作者: z_weining 时间: 2002-10-19 16:15
18题：
先用“有关无关”和“求同求异”的方法判断，
a,或者所有都是随机，或者没有是随机的。原文中，提到了“所有”，也提到了“随机”，所有信息有关。放下。
b,试验中的细菌是一般形式。只说了“细菌”有关，所以试无关选想。
c,如果细菌中的所有变异都是随机的，那么其他生命的变异也是随机的。只提到了“随机”试有关的，“其他生命的变异”原文中没提。所以无关。
d,发生变异的环境对基因变异没有影响。有关，放下。
e,营养和细菌的营养一样，显然无关。
可恶的是，a和d都表示求同，但a 明显要比d好，因为：
a说的是“或者所有都是随机，或者没有是随机的。”，但是原文已经告诉我们，“genetic mutations occurred at random in the populations”，所以，就排除了a的后半句话了，那么就只剩下前半句的原文结论了。
请看：drink two or three alcoholic beverages a day and thereby exceed the caloric intake necessary to maintain their weight ------------consumed in excess of those with which the body needs to be provided to maintain its weight are normally stored as fat and the body gains weight.作者: 1stzhang 时间: 2002-10-26 23:14
2. asking: resolve the apparent discrepancy but what is the apparent discrepancy ?
The trick part: Two group of people under same condition which is all consumed excess food but the real result is different because one group pf people regularly drink two or three alcoholic beverages a day.
So. Answer B.
THe answer A is wrong because it is inconsistent with premise, saying that This group of people (avoid exceeding the caloric intake)necassary to maintain their weight (by decreasing caloric intake from other sources).The premise: The intake of the both group is exceeded.
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