(D) says, "there is no evidence that the behavior of some of the children was unaffected by additives"
The problem with (D) is that the conclusion could still be true even if none of the children were unaffected by additives, so the argument does not need to provide evidence that some children were unaffected. Thus, we can get rid of (D).
(B) says, "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"
With (B), having a control group is one possible way to reduce potential alternative causes. Perhaps the additives didn't have anything to do with the change in behavior and such behavior naturally changes over time. In order to eliminate this possibility, we would need to study a group of children whose diet was not changed as well. Thus, (B) is the correct answer choice.
Does this make sense? Let us know if you have any other questions!
