In 2015, astronaut Scott Kelly made himself a guinea pig for NASA's "twins study," designed to see what spaceflight does to the human body. It was for all the people who dream of human journeys to Mars and other destinations in space. Kelly rode a rocket into space and spent nearly a year on the International Space Station in low Earth orbit, while his identical twin brother, Mark Kelly, served as the comparison subject and stayed on Earth's surface.

The full results, published Thursday in the journal Science, showed that Scott Kelly experienced numerous physiological and chromosomal（染色体的）changes during his long stay in orbit, including changes in gene expression. His immune system went on high alert, both when he went to space and upon returning to Earth. His body acted as if it were under attack.

One of the most dramatic findings concerned epigenetics（实验胚胎学） — how genes are turned on or off to produce proteins. Gene expression changed in both Kellys during the study but in significantly different ways. The study found that more than 90 percent of Scott Kelly's gene expression changes returned to normal when he landed on the surface. His telomeres（染色体端粒）, structures which break over time as part of the natural aging process, lengthened in space. But that didn't necessarily mean being younger, the study found, because most telomeres shortened dramatically when he returned to Earth.

Months later, tests showed that slight changes in telomeres length still remained and left some influence on Scott Kelly, which means he and his brother are no longer identical twins. "He might be at some increased risk for cardiovascular disease or some types of cancer," said Susan Bailey, a biologist at Colorado State University who led one of the investigations in the study.

However, the researchers, echoing what NASA has suggested previously, said the twins study turned up no showstoppers — no shocking health consequences that would surely prevent a human mission to Mars or similar long-duration mission.

【小题1】What does the underlined expression "a guinea pig" in Paragraph 1 mean?

A．a person who is strong enough to become an astronaut.

B．a person who is a leader of space exploration.

C．a person who is picked out for a scientific experiment.

D．a person who is faced with the harsh effects of space flight.

【小题2】What happens to an astronaut's body during a space flight?

A．The permanent changes in gene expression.	B．The aging of cells.
C．The lengthened telomeres.	D．The failure in immune system.

【小题3】What can we learn from the passage?

A．Scott Kelly will appear younger because his telomeres lengthened in space.

B．A long-duration spaceflight will be banned because of the damage to health.

C．Scott Kelly's gene expression changes were normal when he returned to the surface.

D．Scott Kelly might be more likely to develop cardiovascular disease or some types of cancer.

【小题4】What can be a suitable title for the text?

A．Gene expression: Physiological Change of Proteins.

B．Space Experiment: Changes in Both Kellys.

C．Mars Exploration: an Unstoppable Human Mission.

D．Shortened Telomeres: the Killer of Astronauts.