In his book,he explains both ______ it means to be a scientist and ______ science works.

The first thing ____ you should do is to learn all the new words by heart.

________ is known to all is that effective measures should be taken to deal with the problem of the over­exploitation of our resources.

It is pretty well understood __________controls the flow of carbon dioxide in and out the atmosphere today.

The clever boy made a hole in the wall, _______ he could see _____ was going on inside house.