Read the following passage and mark the letter A, B, C or D to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.

Large forests are important to us in many ways. They give us wood (26) ________ building and heating. They are a home for many kinds of plants and animals. And for many city people, forests are a place to go for a vacation. People can (27) ________ about nature there. They can enjoy fresh air and sleep in a(n) (28) ________ place. But there is one more reason (29) ________ forests are important to everyone. The leaves on the trees in a forest help to clean the air. Dirty air is a serious problem in many (30) ________ of the world. Without our forests, this problem might be much worse.