问题补充：
- Can you go to row a boat with us this afternoon?
- Im afraid not. I look after my little sister because my mother is out.
A. can
B. may
C. would
D. have to
答案：
A. can
B. may
C. would
(答案→)D. have to
解析：句意：--今天下午，你能与我们去划船吗？--我恐怕不能。因为我的母亲外出了，我不得不照顾我的小妺妺。分析由于自己不能去参加活动，体现不得不。 故选D
