题目内容：
Should the U.S. halt human space flight?The Columbia accident has revived the debate on whether the rewards of human space exploration are worth the risks.
No. I was a teacher when men first landed on the moon in 1969, and I remember how it moved my students and this country.___________(46) And we haven't ventured outward since then. That's 30 years too long! America's human space-flight program is adrift, with no clear vision or goals beyond the completion of the International Space Station.
I want NASA to establish a phased series of goals over the next 20 years, including human visits to asteroids that cross the Earth's orbit, establishing a research and living facility for humans on the moon, and human expeditions to the surface of Mars and its moons.___________(47)
An astronaut is today's Christopher Columbus, who sailed into the unknown and discovered the Americas. The knowledge we gain from having actual people exploring can never be replaced by robots.___________ (48) Robots are useful, but humans can do things that robots can't.
The real obstacle we face in overcoming the drift in the nation's human space-flight program is not technological and it's not financial.___________ (49)The lesson from the Columbia accident is not that humans don't belong in space.___________ (50)
A．Instead, we should honor the memory of the lost astronauts by pushing our exploration of space future.
B．Astronauts are key to this expanded exploration.
C．It's the lack of commitment to get started.
D．Until then, we should stop risking people's lives by sending them into space.
E．It is now more than 30 year since the last American left the surface of the moon and returned to Earth.
F．Our ability to send humans into space and have them return gives us amazing information about ourselves and our universe.
请选择（46）处的最佳答案．
