请教各位大牛

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.（C）

(E) If not A, then not D.

why???

昏到，我宁愿来长点的！！！