问题补充：
Youbuy Tom a present if he invites you to his birthday party.
A. had better
B. would like
C. are able to
D. must.
答案：
(答案→)A. had better
B. would like
C. are able to
D. must.
解析：句意：如果汤姆邀请你参加他的生日聚会，你最好给他买个礼物。you had better do sth,你最好做···，提建议或劝告。would like to do sth,想要做某事，be able to do 能够做某事，must do必须做某事。结合句意及结构，故选A。
