For most of the 20th century, there has been life on Mars, or, at least in the minds of the Earthlings, who live on its closest planetary neighbor. But could life exist on Mars, given that it is a very (47) world from what it was 3.8 billion years ago It has (48) no atmosphere. It has a barren suce, which is bathed in ultraviolet (49) from the sun, and there is no liquid water on the suce of the planet (although there are ice caps at its poles). Mars had moved from a warm, wet place -- a likely home for life -- to a dead world of dust storms, (50) and vast canyons. It happened because its carbon dioxide atmosphere could not remain (51) in a wet environment. Carbon dioxide reacted with rocks, and it was rapidly absorbed by water, where it became solid carbonates (碳酸盐) and (52) to the bottom of seas. It is a greenhouse gas, which (53) heat from the sun. As it disappeared from the Martian atmosphere, the planet began to cool. Its atmosphere grew thinner and all the water on the suce (54) This is a fate that the Earth escaped because of water vapor and other greenhouse gases, which are (55) present in its atmosphere.
And this is not necessarily the end of the story. Mars could one day be a warm, wet world again, if sufficient greenhouse gases were (56) into the atmosphere -- a task our descendents may one day set themselves when they begin to colonize the solar system.

• A) frozeI) radiation
• B) differentJ) distinguished
• C) steady K) virtually
• D) traps L) earthquakes
• E) volcanoes M) sank
• F) adequate N) constantly
• G) stable O) confines
• H) released