For years scientists have been concerned about the effects of air pollution on the earth's natural conditions. Some believe the air inside many houses may be more dangerous than the air outside. It may be one hundred times worse.
Indoor air pollution can cause a person to feel tired,to suffer eye pain, headache and other problems.Some pollutants(污染物)can cause breathing disorders,diseases of blood and even cancer. Most scientists agree that every modern house has some kind of indoor pollution.People began to notice the problem in the early 1970s.It is said that when builders began making houses and offices they did not want to waste energy.To do this,they built buildings that limited the flow of air between inside and outside.They also began using man-made building materials.These materials are now known to let out harmful gases.As the problem became more serious, scientists began searching for a way to deal with it.They discovered a natural pollution control system for building－green plants.Scientists do not really know how plants control air pollution.They believe that a plant's leaves absorb or take in the pollutants.In exchange the plant lets out oxygen through 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 metres of space.Studies of different plants show that each absorbs 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.
77.What is the topic of the passage?
A.Air pollution.           
B.Green plants indoors.
C.Indoor air pollution.         
D.Environment protecting.
78.What makes indoor air pollution more harmful than that of the air outside?
A.The limited flow of air indoors.
B.Man-made building materials.
C.Fewer green plants inside houses.     
D.The energy-saving materials indoors.
79.The most effective way to clean the air is to use different kinds of plants because______.
A.they can make houses pettier and healthier
B.they may let out oxygen which people need
C.they can take in all kinds of indoor pollutants  
D.they can absorb different harmful indoor chemicals
80.Which of the following is implied but NOT stated?
A.keep windows open and let fresh air in.
B.Scientists have a clear idea how plants control air pollution.
C.Oxygen is let out through the leaves and roots of the plants.
D.Green plants are a natural pollution control system for building.