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
网友回答
(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。