单项选择题

A．Because it is interesting and creative.
B．Because it has a big effect on our lives.
C．Because she can make big money with it.
D．Because she can graduate more easily.
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单项选择题
The major difficulty in developing space travel is that ______.
A．there is no clear destination in space
B．it is too risky and expensive for people to go beyond the orbit
C．the current space propulsion systems can not meet the needs for the long space travel
D．there is not enough energy for spacecraft to gain velocity
单项选择题
Which of the following is consistent with the passage
A．Current rockets are vulnerable and unstable.
B．New propulsion system has been developed.
C．People consider laser beams as a possible solution to developing new propulsion system.
D．At present, nuclear energies can be used in propulsion systems.
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