2．Are we humans alone in space？Scientists have been working on this question for a long time．And recently，new （16）A offer more hope that some form of life was on Mars in the past and perhaps is still there（17）B．
According to a new report in the book Science，the Mars Explorer Curiosity has found more methane（甲烷） in （18）D air．It might suggest the possibility of life．
During the last 20months，Curiosity has found methane many times，and four of them，it tested at a level 10times （19）C than usual．"It's a great increase，"said Christopher Webster，the lead scientist．
This new finding has made a big change．In September 2013，Curiosity found almost（20）B sign of methane on Mars．Just weeks later，however，Curiosity （21）A the gas．
If there's methane in Mars'air，where is it coming from？Scientists aren't sure．Methane is also found in Earth's air and comes from animal and plant life，as well as from the environment（22）D．
"There are many（23）A sources（来源），such as actions between water and rock，"guessed Sushil，a scientist of the Curiosity team．
Also，for the first time，Curiosity found some other organisms（有机物） in the rocks of Mars．Organisms are thought to be the necessary things to form life．Scientists aren't sure where these organisms come from．They could have formed on Mars （24）D landed on Mars by meteorites．
For the next three years，Curiosity will （25）C looking for more organisms in rocks and exploring Mars．There is still a long way to go!
| 16．A．discoveries | B．inventions | C．books | D．magazines |
| 17．A．in time | B．at present | C．at once | D．in future |
| 18．A．Earth's | B．Sun's | C．Moon's | D．Mars' |
| 19．A．high | B．low | C．higher | D．lower |
| 20．A．all | B．no | C．every | D．much |
| 21．A．picked up | B．put up | C．gave up | D．opened up |
| 22．A．himself | B．themselves | C．herself | D．itself |
| 23．A．possible | B．necessary | C．certain | D．important |
| 24．A．and | B．but | C．so | D．or |
| 25．A．stop | B．practice | C．keep | D．remember． |
