题目内容：

    The United States space agency, NASA. says its Mars exploration vehicle recently recorded a high level of methane (甲烷) gas on the planet. The discovery is exciting because the presence of methane gas could support the case for life on Mars.

NASA's Curiosity vehicle recently recoded the largest level of methane ever measured during its seven-year Mars mission.

Methane has no color or smell. A special instrument on Curiosity's Man Science Laboratory recorded the increased gas level. The device measures levels of chemical elements and gases in the Marian atmosphere. In addition to methane, the instrument can record levels of water and carbon dioxide.

Nearly all the methane gas found in Earth's atmosphere is produced by living thing. It usually comes from animal and plant life. But it can also be formed by geological(地质的) processes, such as interactions. between rocks and water.

It was not the first time Curiosity had found methane gas in the Martian atmosphere. About a year ago, NASA announced that Curiosity had discovered sharp seasonal increases in the gas. This time, NASA said the measured methane gas level was clearly larger than any others observed in the past. "It's exciting because microbial (微生物的) life is an important source of methane on Earth," NASA said in a statement announcing the discovery.

However, Curiosity's team carried out a follow-up methane experiment that showed a sharp drop in levels of the gas. That number was "close to the background levels Curiosity sees all the time." NASA said. The rise and fall of the methane gas levels left NASA scientists with more questions than answers.

"The methane mystery continues," said Ashwin Vasavada. Curiosity's project scientist, "We're more motivated than ever to keep measuring and put our brains together to figure out how methane behaves in the Marian atmosphere."

1.What do we know about methane go?

A.It is a must to living things. B.It can be found everywhere.

C.It smells like carbon dioxide. D.It's mainly from biological activity.

2.What do the explorations on Mans tell us?

A.Humans have known much about Mars.

B.There will be more methane gas on Mars.

C.There is no progress in the 7 year's research.

D.The level of methane gas on Mars is not stable.

3.What does the discovery mean in NASA's view?

A.There may exist life on Mars.

B.Methane gas on Mars has a sudden increase.

C.The mystery about Mars will be solved soon.

D.They're known how methane as is formed on Mans.

4.How do the NASA's scientists feel about their future exploration?

A.Aimless. B.Confident.

C.Stressed. D.Serious.