Scientists have been worried for years about the 　　　of air pollution on the earth’s natural conditions.

　 A．effect　　 B．reason　　　 C．account　　 D．cause

 For years, scientists have been worried about the _______ of air pollution on the earth’s natural conditions.

A. effect         B. result           C. account          D. cause  

For years scientists have been worried about the ___________ of air pollution on the earth’s natural conditions.

　 A. effect B. result C. account D. cause

Section B
Directions: Complete the following passage by using the words in the box. Each word can only be used once. Note that there is one word more than you need
A. agree          B. suffer        C. system       D. notice        AB. through  
AC. dangerous     AD. space       BC. limited     BD. absorbs      CD. let out
For years scientist have been worried about the effects of air pollution on the earth’s natural conditions. Some believe the air inside many houses may be more    41    than the air outside. It may be one hundred times worse.
Indoor air pollution can cause a person to feel tired, to    42     eye-pain, headache and other problems. Some pollutants can cause breathing disorders, diseases of blood and even cancer. Most scientists    43    that every modern house has some kind of indoor pollution. People began to    44    the problem in the early 1970s, when it was that builders began making houses and offices which did not waste energy. To do this they built buildings that    45    the flow of air between inside and outside. They also began using man-made building materials. These materials are now known to    46    harmful gases. As the problem became more serious, scientists began searching for a way to deal with it. They discovered a natural pollution control    47    for building green plants. Scientist do not really know how plants control air pollution. They believe that a plant’s leaves take in the pollutants. In exchange the plant produces oxygen    48    its leaves and through its tiny organizations on its roots. Scientists suggest that all buildings should have one large plant or several small plants inside for every nine square meters of space. Studies of different plants show that each    49    different chemicals. So the most effective way to clean the air is to use different kinds of plants. Having green plants inside your house can make it a prettier and healthier place.