单选题 A large group of hyperactive children whose regular diets included food containing large amounts of additives was observed by researchers trained to assess the presence or absence of behavior problems. The children were then placed on a low-additive diet for several weeks, after which they were observed again. Originally nearly 60 percent of the children exhibited behavior problems; after the change in diet, only 30 percent did so. On the basis of these data, it can be concluded that food additives can contribute to behavior problems in hyperactive children.
The evidence cited fails to establish the conclusion because

A、 there is no evidence that the reduction in behavior problems was proportionate to the reduction in food-additive intake.
B、 there is no way to know what changes would have occurred without the change of diet, since only children who changed to a low-additive diet were studied.
C、 exactly how many children exhibited behavior problems after the change in diet cannot be determined, since the size of the group studied is not precisely given.
D、 there is no evidence that the behavior of some of the children was unaffected by additives.
E、 (E) the evidence is consistent with the claim that some children exhibit more frequent behavior problems after being on the low-additive diet than they had exhibited when first observed.

【正确答案】 B

【答案解析】[解析] 当我们想通过比较来研究某一因素对某件事情的影响时，要在保证其他条件不变的情况下仅改变这一因素来研究它，只有这样我们得到的数据才有可能是可信的。段落中的论据却没有这样做，根据段落中提供的证据，我们不难推出，它是以饮食中包含有较多添加剂的儿童在不改变饮食的情况下，过了几周以后，有行为问题的儿童的比率不变为前提的，也就是说该证据没有用发展的眼光来看待儿童的行为问题。因此当我们用发展的眼光来看待儿童的行为问题时，我们将无法知道若不改变饮食，那些儿童的行为是否也会发生变化。由此分析我们可以得知(B)是正确选项。(A)、(C)和(D)都不是这个证据应该解决的问题；(E)引入了新的比较，因此也是错误的。