on an exercise bike. Replace your office chair with a therapy(治疗) ball: this uses more muscles and more energy than a normal chair, because you have to support your back and work to keep balanced. But whatever you choose, know this. It is clear: be careful of your chair.

28. The main purpose of writing this passage is to __________.

A. suggest sitting less.

B. advise exercising often.

C. avoid gaining weight.

D. escape standing more.

29. From this passage, we know that _________.

A. less exercise will reduce the chance of having disease.

B. taking exercise regularly but sitting more will do good.

C. standing burns nearly as much energy as sitting still does.

D. energy-burning activities may help you enjoy good health.

30. The underlined word “This” in Paragraph 4 refers to __________.

A. gaining weight.

B. burning energy.

C. sitting too much.

D. standing more.

31. A therapy ball is mentioned in the last paragraph to _________.

A. tell us to walk while working in the office.

B. suggest a way to settle the sitting problem.

C. support our back and keep us balanced.

D. provide a better cure for our backache.

D

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 anti-tumor 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