There are several advantages in planting trees：they firm the soil，soak(渗透) up extra water and take carbon dioxide out of the atmosphere. However，it now turns out that planting trees could add to global warming. We all know that tree roots do a great job of keeping soil firmly on the ground and out of the wind's power. The problem is that some of the dust clouds play an important part in soaking up carbon dioxide.

Huge dust storms blow out over the oceans from  dry parts of North Africa and Central Asia. Tons of dust are lifted and left as a thin film over the ocean surface.

Dust from China is carried east and left in the Pacific Ocean. If a tree­planting programme there is successful and the dust supply reduced, the net result may be that less carbon dioxide gets locked away in the ocean.

Andy，an environmental scientist has spent the past few years studying dust and says his work shows clearly the complexity of the system. For this reason the need is to focus on cutting carbon dioxide giving off rather than doing anything else.

Robert, an American scientist, has  shown that when native grassland areas are invaded by trees, carbon is lost from the soil. “We are studying why the soil carbon disappears，but one theory is that trees do a lot more of their growing above ground compared to grass, so less carbon goes directly into the soil from trees.” said Robert.

In wet areas of the world, the gain from trees absorbing carbon dioxide above ground seems to be outweighed by the loss of carbon from the soil below ground.

Countries that plan to fight against global warming by planting trees may have to think again. Solutions to environmental problems are often more complex than they first appear，and understanding the Earth's climate is a very great challenge.

60．People usually hold the opinion that ________.

A. huge dust storms can destroy carbon dioxide

B. planting trees could reduce global warming

C. huge dust storms can destroy the oceans on the Earth

D. planting trees is the only way to control huge dust storms

61．Andy, an  environmental  scientist, believes  that________.

A. environmental problems are more complex than expected                 

B. trees shouldn't have been planted in dry places

C. dust plays a more important part than trees

D. carbon dioxide is harmful to everything

62．Robert's experiment proves that ________.

A. trees absorb more carbon than grass

B. carbon can turn grass into dust

C. less carbon can make trees grow faster

D. grassland areas should be covered by forests

63．The best title for this passage is “________”．

A. The importance of planting trees

B. The dust clouds soak up carbon dioxide

C. Is it really useful to plant trees to reduce global warming?

D. Why the soil carbon disappears

A great person is always putting others’ interests _________ his own.

Pretoria．South Africa—Hundreds 0f swimmers Play in the water along a South African beach．Suddenly a huge shark appears offshore and heads for the bathers．Before it can get close to the swimmers，the shark，for some mysterious reason，starts swimming parallel to the beach．Then it turns away from shore and heads out to sea，leaving the swimmers untouched．

Such a scene will soon become common along shark infested（鲨鱼出没）shores．An electrical engineer from the University of Pretoria has developed an electrical system that repels（击退）sharks by controlling their movements．Confused by this action，the sharks end up turning away from the beach．

In this unusual shark repellent method，an insulated（绝缘）cable is buried on the bottom of the sea around a beach from which people swim．When an electric current is sent though the cable，an electric field is produced in the water above the cable．

When the shark swims into the electric field，nerve cells just below the shark’s skin detect the field．The electricity produces a reflex（反射）action in the shark’s muscles，causing the dangerous animal with sharp teeth to swim parallel to the cable．Contused by increasing loss of control of its muscles，the shark suddenly changes its direction and heads seaward，away from the electric field．   

In a test of the invisible new shark-repellent system，a cable was placed around a beach in St．Lucia an island in the tropics．A protective net，which had been installed around the beach a long time ago to keep sharks from swimmers，was left in place during the experiment．

When the electric field was 1eft on for more than a year and a half，no sharks were found in the nets．However,when the power was turned off，89 sharks ended up in the nets．

Using the electric wall method instead of nets to protect bathers would avoid the needless death of many sharks，say biologists．Currently，thousands of sharks die each year when they become trapped in protective nets that surround holiday resorts in Africa and Australia．About 10 to 15 humans are killed by sharks annually．

1.In the first passage，why doesn’t the shark attack the swimmers？

A．Because they are many swimmers in the water．

B．Because the shark doesn’t want to eat the swimmers。

C．Because of some mysterious reason from the God．

D．Because there is an invisible electric field confusing the shark．

2.An electrical engineer from the University of Pretoria has developed         ．

A．A protective net                     B．A new shark-repellent system

C．The electric field                       D．An insulated cable

3.An electric field is produced in the water above the cable          ．

A．only when the shark swims into the electric field

B．an insulated cable is buried on the bottom of the sea around a beach from which people

swim

C．only when an electric current is sent though the cable

D．only when the sharks are trapped in protective nets

4.Which of the following is NOT TRUE according to the passage?

A．Protective nets had been installed around the beach earlier than the new shark repellent system．

B．This new kind of shark repellent system wilt take place of the protective nets．

C．Thousands of sharks die and about 10 to 15 humans are killed by sharks each year in Africa and Australia since this new system was Put into use．

D．Using the electric wall method instead of nets to protect bathers would avoid the needless death of many sharks．

A great person is always putting others' interests _________ his own．

A．below              B．above               C．in                D．on