I could have turned to Mr. Smith for help but he too busy then.A. isB. wasC. should beD.

问题补充：

I could have turned to Mr. Smith for help, but he too busy then.

A. is

B. was

C. should be

D. has been

答案：

A. is

(答案→)B. was

C. should be

D. has been

解析：本题句意“我本可能救助鱼史密斯先生的，但是他那时实在太忙了”。本句中的could have turned说明了动作发生在过去，有but转折连词，导出了后句的事实性。