用所给词的适当形式填空.1.—May I go out now,Dad?—No.You must___(let) your mother know first.

用所给词的适当形式填空.1.—May I go out now,Dad?—No.You must___(let) your mother know first.
2.He is active in ___(sing) in the competition.
3.They left without ___(say) goodbye.