Mike often exercises half an hour in the morning.A. atB. ofC. withD. for

问题补充：

Mike often exercises half an hour in the morning.

A. at

B. of

C. with

D. for

答案：

A. at

B. of

C. with

(答案→)D. for

解析：at在时间空间的一点；of关于，……的；with伴随，用某种工具；for为了，对于。句意：麦克经常在上午锻炼半个小时。For后面常接一段时间，表示动作持续的时间长短，故选D。