Why ____ you keep troubling your mother with such question while he is busy . 

It’s required that the students______ mobile phones in their school, so seldom ______ them using one. 

But for their help，we         the program in time.

He had dreamed of going there for a long time. But he _______ he would have such an unpleasant experience.

Every solid object ______ reflect a sound，varying according to the size and nature of the object．