radicals (自由基). Free radicals can damage DNA and lead to diseases such as cancer.

　 However, bats are not affected by free radicals produced during flight. The reason is that bats carry special genes to repair any damaged DNA.

　 Scientists think that because bats fly and produce free radicals, they've evolved to carry genes that shaped their special immune system.

　 "We're thinking that the evolution of flight led to a sort of effect, influencing not only the immune system but also things like aging and cancer," says Chris Cowled, of the Australian Animal Health Laboratory. "We think we've really found something special."

　 Researchers believe that this discovery might one day be key to helping humans fight diseases such as cancer that attack the immune system and damage DNA.

4. What's true about bats according to the passage?

A. They carry many deadly diseases like SARS so it's easy for them to get sick

B. Bats are the only animals that can tell directions easily

C. Bats are among the mammals that can truly fly

D. Researchers studied bats to help save people

5. According to researchers, bats can fight off many deadly diseases mainly because ________.

A. they have developed special immune system through evolution

B. they are able to fly, which burns a lot of energy

C. free radicals in their bodies can damage DNA and cause diseases

D. they carry no special genes to repair damaged DNA

6. The passage is mainly about ________.

A. bats and dangerous diseases

　　B. bats and their special immune system

C. free radicals and immune system

　　D. ways to cure cancer

　　C

Ask someone what they have done to help the environment recently and they will almost certainly mention recycling. Recycling in the home is very important of