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
网友回答
(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。