How he get out of the prison a mystery.A. could; isB. can; isC. was able to; remainsD. a

问题补充：

How he get out of the prison a mystery.

A. could; is

B. can; is

C. was able to; remains

D. able to; remain

答案：

A. could; is

B. can; is

(答案→)C. was able to; remains

D. able to; remain

解析：考查情态动词。句中be able to其过去式表示成功的做某事，从句作主语，谓语动词用单数形式，故选C。句意：他怎么成功的出监狱还是个迷。