刚开始接触CR。各位大虾，能否给小弟解释下该CR题？

If A, then B.

If B, then C.

If C, then D.

If all of the statements above are true, which of the following must also be true?

(A) If D, then A.

(B) If not B, then not C.

(C) If not D, then not A.

(D) If D, then E.

(E) If not A, then not D.

选C。考得就是充分条件和必要条件以及取非。LZ看看置顶帖学习一下吧。考试的时候不会有这么简单的题.........