Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 33 to 37.

The (33) _____ of the earth is caused by exhaust gas from automobile engines, factories and power stations. Carbon dioxide goes up into the atmosphere, and it forms a kind of factories that keeps or rather allows the sunshine in but stop the earth’s heart (34) _____ getting out. It works like a greenhouse, that’s why we call it the Greenhouse Effect.

Because of this effect, the earth is getting warmer all the time. This (35) _____ in temperature will cause big changes to the world's climate. The sea level will rise, the ice covering the poles will melt and cause the sea level to rise.

The second problem is the (36) _____ of the ozone layer. Certain chemicals float up to the sky and react with the ozone layer, and they make holes in it. Because of these holes the ultraviolet rays can shine directly to the earth. Many people are now starting to suffer from skin cancer.

The sulfurous smoke from factories and power stations mixes with rain clouds and gets blown by the wind and then falls as acid rain (37) _____ destroys lakes and forests.

These three problems threaten our environment at the end of the twentieth century. Unless we do something about them quickly, we, human race, may disappear from the face of the earth.

Điền vào số (37)

A. it  

B. which     

C. what       

D. they