单项选择题

The passage is mainly about ______.

A．the development of space' technology
B．the obstacles and prospects of space transportation
C．the public interests in space travel
D．the growth of space business

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单项选择题

According to Rob Reiner, the year from zero to three is ______.

A．a critical stage to develop human nerves
B．extremely important for a person's development
C．studied carefully for government to make policies
D．worth more efforts in education than that in later years

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