At last the mother had no choice but her son herself though she was quite busyA. to pic

问题补充：

At last the mother had no choice but her son herself, though she was quite busy．

A. to pick up

B. pick up

C. picking up

D. picked up

答案：

(答案→)A. to pick up

B. pick up

C. picking up

D. picked up

解析：固定结构：had no choice but to do sth 别无选择只有…。 最后妈妈别无选择只有背起孩子尽管她很忙。