21.The moment we heard the news of the earthquake, we began to ________ the work to rescue those who were trapped in the building.
There are ______ students in the school . But I don’t know _______ the girls among them .
–Does he know how to work out the problem ? -Yes , he has ________ a good idea to solve it .
High school students should be given enough sleeping time , _______ they are busy with their study .
She told us ______ story that we all forget about the time .
She told me _______ a noise in the hall .