[单项选择] Unless we spend money to spot and prevent asteroids (小行星) now, one might crash into Earth and destroy life as we know it, say some scientists. Asteroids are bigger versions of the meteoroids (流星) that race across the night sky. Most orbit the sun far from Earth and don’’t threaten us. But there are also thousands of asteroids whose orbits put them on a collision course with Earth. Buy $ 50 million worth of new telescopes right now. then spend $ 10 million a year for the next 25 years to locate most of the space rocks. By the time we spot a fatal one, the scientists say, we’’ll have a way to change its course. Some scientists favor pushing asteroids off course with nuclear weapons. But the cost wouldn’’t be cheap. Is it worth it Two things experts consider when judging any risk are: 1) How likely the event is; and 2) How bad the consequences if the event occurs. Experts think an asteroid big enough to destroy lots of life might strike Earth once every 500,000 ye
A. Optimistic.
B. Critical.
C. Objective.
D. Arbitrary.

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