Now one of the most serious problems manypeople are worried about _____ on the lack of adequate housing space.

In my view, biology is ________ chemistry, but a little easier than mathematics.

There is a big tree ________ the classroom.

Who is it up ____ decide whether to go or not?

The moment we heard the news of the earthquake, we began to ______ the work to rescue those who were trapped in the building. [ ]