Several studies have found that the coronary patients who exercise most actively have half or less than half the chance of dying of a heart attack as those who are sedentary.

(A) have half or less than half the chance of dying of a heart attack as those who are sedentary

(B) have half the chance, or less, of dying of a heart attack than those who are sedentary do

(C) have half the chance that they will die of a heart attack, or less, than those who are sedentary do

(D) are at least fifty percent less likely to die of a heart attack as those who are sedentary（E）

(E) are at least fifty percent less likely than those who are sedentary to die of a heart attack

请帮忙解释一下B和C如何排除?

B, C do not have any grammar errors. They violate the effectiveness rule.

'at least 50% less likely to do' is more efficient.

Refer to OG-266.

266. In a recent poll, 86 percent of the public favored a Clean Air Act as strong or stronger than the present act.

(A) a Clean Air Act as strong or stronger than

(B) a Clean Air Act that is stronger, or at least so strong as,

(C) at least as strong a Clean Air Act as is

(D) a Clean Air Act as strong or stronger than is（E）

(E) a Clean Air Act at least as strong as

E, the best choice, is concise, clear, and idiomatic. Choices A, B, C, and D may be faulted for constructions that are cumbersome, unnecessarily wordy, or unidiomatic. Choices A and D require as strong as instead of as strong. Similarly, B is missing than after stronger, and so should be as. In C and D, is should be dropped. Even with revisions, these choices are more wordy and awkward than the best answer.

But, take precaution. 'at least' sometimes creates redundancy. see OG-59.

59. In 1978 only half the women granted child support by a court received the amount awarded; at least as much as a million and more others had not any support agreements whatsoever.

(A) at least as much as a million and more others had not any

(B) at least as much as more than a million others had no

(C) more than a million others had not any

(D) more than a million others had no（D）

(E) there was at least a million or more others without any

D, the best choice, is idiomatic, clear, and concise. Both A and B incorrectly use much rather than many to describe the countable noun others; much should be used with uncountable nouns such as “joy” and “labor.” Even if this error were corrected, though, A and B would still be wrong. Because more than x necessarily includes the sense of at least as many as x, it is redundant and confusing to use elements of both expressions to refer to the same number of women. In A and C, not any support agreements is wordy and awkward. Like A and B, E redundantly uses both at least and more, and it incorrectly links the singular verb was with the plural subject others.