求助一道LR evaluate 题目~~

Eating garlic reduces the levels of cholesterol and triglycerides in the blood and so helps reduce the risk of cardiovascular disease. Evidence that eating garlic reduces these levels is that a group of patients taking a garlic tablet each day for four months showed a 12 percent reduction in cholesterol and a 17percent reduction in triglycerides; over the same period, a group of similar patients taking a medically inert tablet showed only a 2percent reduction in triglycerides and a 3percent reduction in cholesterol.

It would be most important to determine which one of the following in evaluating the argument?

A.   whether the garlic tablets are readily available to the public.

B.     What the diet of the two groups were during the period.

C.     What effect taking the garlic tablets each for a period of less than four months had on the levels of cholesterol and triglycerides.

D.    Whether large amounts of garlic are well tolerated by all patients.

E.     Whether the manufacturer of the garlic tablets cites the study in its advertising.

The answer is B, I chose D, can't see why B is better than D, why B not D????????

THANKS A LOT!!

问题是要确定 ：garlic是不是降低胆固醇的原因，所以需要排除其他可能性 比如是否diet中的其他东西在起作用，所以需要知道 B。 是否tolerate 没有关系。