Life on Mars
Perhaps more than anything else, scientists are eager to find out if Martian life existed in the past—or still exists.【61】telescopes first zoomed in【62】Mars in the 17th century, people have conjured up a wild【63】of images or what Martians might look like. But space probes like the 1997 Sojourner land rover have yielded no evidence of such alien beings.
Most experts agree that if life did at one time evolve on Mars, finding evidence of that life—which would likely take the form. of tiny organisms—won't be easy.【64】, many scientists are optimistic. "We've got organisms on Earth that adapted to life deep【65】the surface in underground water wells, " says Stephen Clifford. "【66】life like that evolved on Mars four billion years ago, there's no reason why it【67】today. "
【68】last year's disappointing losses, the future of Martian exploration looks【69】This year, two major
films about fictitious Mars missions—Red Planet and Missions to Mars—are certain to heighten interest in our planetary neighbour. More important, plans for new sets of NASA orbiters and landers—one to launch in 2001 , the other in 2003—are already in the works. Without a doubt, each new mission will inch scientists closer to【70】the mysteries of planet Mars.
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A. When
Before
C. Ever since
D. Ever