Don't worry.They'll come to help us ________.

题目

Don't worry.They'll come to help us ________.
A.in half an hour B.in a half hour C.after half an hour D.after a half hour答案为什么是A不是B

答案

in +时间段,用将来时态
after+时间段,用过去时态.
half an hour 半小时的表达方式.