题目内容：

    Elephants have impressed us for centuries. They are big, clever, and sociable. But what if someone told you that they may also hold the key to fighting cancer (癌症)?

People have been wondering why elephants do not develop cancer, even though they have lifespans (寿命) that are similar to humans, living 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) from developing. To be precise, they found 20 copies of an anti­tumor gene called TP53 in elephants. Most other species, including humans, only carry one copy.

According to the research, which was recently published on the online science network BioRxiv, the extra copies of the gene improved the animal's sensitivity to DNA damage. This 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，” study author Dr. Vincent Lynch told The Guardian. If every living cell (细胞) has the same chance of becoming cancerous, large creatures with long lifespans like whales and elephants should have a greater risk of developing cancer than humans and mice do. 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”. Evolutionary (进化的) 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 (抵抗) as elephants. This means researchers could use the discovery to develop new treatments that can help stop cancers from spreading or even developing in the first place.

“Nature has already figured out how to prevent cancer, ” said Joshua Schiffman, an oncologist at the School of Medicine, University of Utah, US.

“It's up to us to learn how different animals tackle (处理) the problem so we can use those strategies to prevent cancer in people.”

1.Why are elephants unlikely to develop cancer?

A. They have a large body size.

B. Their genes suffer no DNA damage.

C. Certain genes in their body kill existing tumors.

D. They carry many genes that keep tumors from developing.

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

A. A risk of deadly tumors. B. Human behavior.

C. Extreme weather. D. Cells killing themselves.

3.What does the underlined expression “This phenomenon” in the sixth paragraph refer to?

A. Larger animals have protection from TP53.

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

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

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

4.We can conclude from the last three paragraphs that ________.

A. humans are expected to stop cancer in the near future

B. depending on nature is not enough to fight against cancer

C. this new treatment is more effective than the present ones

D. the TP53 genes have proven useful in stopping cancer in mice