问题
选择题
His mother looked very tired，for she _____ the house herself all day.
A．had cleaned
B．has been cleaning
C．had been cleaning
D．has cleaned
答案
答案：C
考查完成进行时。根据关键词all day可知应该使用完成进行时。再根据上文的looked可知应该使用过去完成进行时。