补全短文The Columbia accident
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 havent ventured outward since then. Thats 30 years too long！ Americas 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 Earths 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 todays 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 cant.
The real obstacle we face in overcoming the drift in the nations human space-flight program is not technological and its not financial.__________(49)
The lesson from the Columbia accident is not that humans dont 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 Its the lack of commitment to get started.
D Until then， we should stop risking peoples lives by sending them into space.
E It is now more than 30 years 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.
答案：E B F C A