Mum clothes so I have to look after my younger sister.A. washesB. is washingC. washedD.

问题补充：

Mum clothes, so I have to look after my younger sister.

A. washes

B. is washing

C. washed

D. was washing

答案：

A. washes

(答案→)B. is washing

C. washed

D. was washing

解析：句意：妈妈正在洗衣服，所以我不得不照顾我的小妹妹。根据答语我不得不照顾我的小妹妹可知我的妈妈正在洗衣服，所以应用现在进行时，be+doing sth的形式，故选B。