Their balloon left the ground ______.

A．asteroids racing across the night sky are likely to hit Earth in the near future
B．workable solutions still have to be found to prevent a violent coming course of aster olds with the earth
C．the worry about asteroids can be left to future generations since it is unlikely to happen in our lifetime
D．while pushing asteroids off course nuclear weapons would destroy the world

A．It sounds practical but it may not solve the problem.
B．Further research should be done before it is proved to be capable.
C．It may create more problems than it might solve.
D．It is a waste of money because a violent coming course of asteroids with the earth is very. unlikely.