Everyone knows that________ is dangerous to play with fire, but________ is difficult is to prevent children from playing with fire.

The librarian told us that we could borrow at most three books __________.

te of his disappointment, he _______ a weak smile.

All flights ___________ because of fog. I’m afraid you will have to stay here for another day.

In the zoo，the number of visitors is tightly to avoid putting stress on the animals.