Shall Mary come and play computer games?--No she has finished her homework.A. whenB.

问题补充：

--Shall Mary come and play computer games?

--No, she has finished her homework.

A. when

B. if

C. unless

D. once

答案：

A. when

B. if

(答案→)C. unless

D. once

解析：本题只要理解了句意就能很容易地选出unless。句意为：“玛丽要来打电脑游戏吗?”“不，除非她完成了作业。”