题目内容：

    People have been wondering why elephants do not develop cancer even though they have lifespans (寿命) that are similar to humans, jiving for around 50 to 70 years.

Now scientists believe they know why. A team at the University of Chicago, US has found that elephants carry a large number of genes that stop tumors (肿瘤) developing. To be precise, they found 20 copies of an anti-tumor gene called TP53 in elephants. Most other species, including humans, only carry one copy.

According to the research, the extra copies of the gene improved the animal’s sensitivity to DNA damage, which lets the cells quickly kill themselves when damaged before they can go on to form deadly tumors.

“An increased risk of developing cancer has stood in the way of the evolution of large body sizes in many animals,” the study author Dr Vincent Lynch told The Guardian. If every living cell has the same chance of becoming cancerous (癌变的), large creatures with a long lifespan like whales and elephants should have a greater risk of developing cancer than humans and mice. But across species, the risk of cancer does not show a connection with body mass.

This phenomenon was found by Oxford University scientist Richard Peto in the 1970s and later named “Peto’s paradox (悖论)”. Biologists believe it results from larger animals using protection that many smaller animals do not. In the elephant’s case, the making of TP53 is nature’s way of keeping this species alive.

The study also found that when the same genes were brought to life in mice, they had the same cancer resistance as elephants. This means researchers could use the discovery to develop new treatments that can help stop cancers spreading or even developing in the first place.

“Nature has already figured out how to prevent cancer,” said Joshua Schiffman, a biologist at the School of Medicine, University of Utah, US. “It’s up to us to learn how different animals deal with the problem so that we can use those strategies to prevent cancer in people.”

1.Why are elephants unlikely to develop cancer?

A.They have a rather large body size.

B.Their genes suffer no DNA damage.

C.Certain genes in their body kill existing tumors.

D.They carry many genes to prevent tumors developing.

2.According to the passage, what has been a risk in the evolution of large animals?

A.Deadly tumors. B.The huge body mass.

C.Cells killing themselves. D.Sensitivity to DNA damage.

3.What does the underlined part “This phenomenon” in Paragraph 5 probably refer to?

A.Larger animals have protection from TP53.

B.The risk of cancer is not related to body size.

C.Larger animals suffer the same risk of cancer as smaller ones.

D.The larger animals are, the bigger risk of cancer they will have.

4.What can be concluded from the passage?

A.Depending on nature is surely enough to fight against cancer.

B.The TP53 genes have proved useful in stopping cancer in people.

C.Humans are expected to prevent cancer with the making of TP53.

D.The extra copies of elephants, genes are used to cure cancer patients.