How he get out of the prison a mystery.
A. could; is
B. can; is
C. was able to; remains
D. able to; remain
网友回答
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。