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。