Trust him please! When he reading it he will return the book to you on time.A. finishB.

问题补充：

Trust him please! When he reading it, he will return the book to you on time.

A. finish

B. will finish

C. has finished

D. finished

答案：

A. finish

B. will finish

(答案→)C. has finished

D. finished

解析：考查时态。句意：请相信他当他读完这本书时，他会及时把这本书还给你。当主句为将来时态时，状语从句用一般现在时态表示将来动作，结合语境可知表示的是将来完成的动作，故用完成时态。选C。