He has been working hard day and night which of course his tiredness.A. accounts forB. a

问题补充：

He has been working hard day and night, which, of course,his tiredness.

A. accounts for

B. answers for

C. applies for

D. stands for

答案：

(答案→)A. accounts for

B. answers for

C. applies for

D. stands for

解析：

这题考查词义辨析：account for是…的原因，answer for为…负责，apply for申请，D. stand for代表，句意是：他白天黑夜的努力工作，这当然是他疲劳的原因了。选A。