高手帮忙解决下

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.

答案为C。可否帮忙解释下？

If A, then B. If B, then C. If C, then D. => If A, then D.

原命题成立，逆否命题成立。 So, If not D, then not A.

多谢，多谢