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. 估计题目没看懂,谁能解释下意思嘛,谢谢了 | 
