fewer young giraffes. A lot of new trees have grown in the Serengeti, but many are a type that giraffes do not like to eat. The researchers found fewer young giraffes today than in the 1970s compared with the number of adult females, a sign that food was in short supply.

　　Dr. Strauss is working on an environmental education program for Tanzania including books for students. These materials will educate Tanzanians and help them to help giraffes. As knowledge grows and changes are made, they hope the giraffe population will increase.

28. What did Dr. Strauss find while studying giraffes in the Serengeti?

A. It was too costly to study giraffes.

B. It was hard to spot giraffes from the air.

C. The number of giraffes had increased slowly.

D. Giraffes lived in smaller areas than in the 1970s.

29.How did the team study the lion-giraffe relationship?

A. By analyzing giraffe droppings. B. By comparing young giraffes with adults.

C. By comparing male giraffes and females. D. By surveying the survival of young giraffes.

30.What are the causes of a reduction in giraffes population in the Serengeti?

A. Lions and parasites. B. Lack of food and poaching.

C. New trees and local markets. D Illegal hunting and natural enemies

31.What is Dr. Strauss doing to help giraffes?

A. She is founding a national park in Tanzania.

B. She is counting giraffes in the entire Serengeti.

C. She is educating students to write about giraffes.

D. She is raising environmental awareness in local people.

D

　　It began as a game: High school and college students studying computer technology figured out they could use personal computers to break into telephone company computers and make free, long-distance telephone calls. These young computer talents soon gained the name "hackers".

　　Police arrested a few hackers, but many went on to even more complex hacking. One of them was arrested for making illegal telephone calls and later he used a phone to change a police officer's credit records to get back at the officer for arresting him. He also used a computer to change his college records to give himself better grades.

　　As hackers gained experience, they began invading computers at banks, airlines and other businesses. In one case a hacker instructed an airline' s computer to give him free airplane tickets.

　　The U.S. government is worried that hackers may break into its networks of defense computers. The government's secrets are easily attacked because thousands of government computers are connected by telephone lines that hackers can get into.

　　In November 1988, a college student entered a U.S. Defense Department computer network called Arpanet. The hacker injected a computer program that made copies of itself throughout Arpanet. Some hackers use viruses to destroy all the data in a computer. But in this case, government officials shut down the network before the program reached every computer in the system. Shutting down the system angered many researchers who were using the computers. The hacker turned himself in to the police and he was charged with a crime.

　　The incident put the spotlight on computer hacking in the United States. Many companies have hired experts to protect their computers from hackers, and many computer experts now advise companies on how to protect their computers.

　　The U.S. government believes foreign governments have hired hackers to try to break into top-secret defense computers.

Experts disagree over whether a computer network can ever be safe from hacking. But in the future, some of the most outstanding minds in the U.S. will be working to frustrate the attempts of