Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.

It was a real race __________ time to get the project completed in such a short time.

We always keep ____ gas in case we run out.

What a pity! He ______the only chance of success.

—Which module is the most difficult in this book?— I think _____ module.