Pretoria, South Africa – Hundreds of swimmers play in the water along a South Africa beach. Suddenly a huge shark appears off shore 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 the shore and heads out to sea, leaving the swimmers untouched.

Such a scene will soon become common along shark-in-fasted (鲨鱼成群的) 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 (绝缘的) cable is buried on the bottom of the sea around a beach where people swim. When an electric current is sent through the cable, an electric field is produced in the water above the cable.

When shark swims into the electric field, nerve cells just below the shark' s skin will sense the field. The electricity produces a reflex (反射作用) action in the shark' s muscles, causing the dangerous animal to swim parallel to the cable. Confused 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 around the beach a long time ago to keep sharks away from swimmers, and a net was left in place during the experiment.

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

Using the electric-wall method instead of net to protect bathers would avoid the needless death of many sharks, said 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．Why doesn' t the shark attack the swimmers according to the passage?

A．Because there are so many swimmers in the water.

B．Because the shark isn’t so hungry as to eat the swimmers.

C．Because of some mysterious power from the heaven.

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 a shark swims into the electric field

B．only when an insulated cable is buried on the bottom of the sea around a beach where people swim

C．only when an electric current is sent through 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 fixed earlier than the new shark-repellent system.

B．The new shark – repellent system will probably replace the protective nets.

C．Thousands of sharks have died each year since this new system was put into use.

D．Using the electric-wall instead of nets would avoid the needless death of many sharks.