单项选择题

With which of the following is the passage primarily concerned

A．The possibility of changing the Martian environment.
B．The challenge of interplanetary travel.
C．The advantages of establishing colonies on Mars.
D．The need to study the Martian ecology.
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单项选择题
According to Christopher McKay, the possibility of transforming Mars______.
A．could occur only in science fiction stories
B．will not begin for hundreds, even thousands, of years
C．is completely impractical
D．could be started in 40 to 50 years
单项选择题
A knowledge of the composition of the weed community ______.
A．is essential to the efficient control of weeds
B．may lead us to be aware of the fact that the soil contains enormous quantities of seeds of numerous species
C．helps us to have a good idea of why seeds can lie dormant for years
D．provides us with the means to identify weeds at the seedling stage
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