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
The best alternative title for the passage will be______.
A．A Study of Weed Communities
B．The Importance of Studying How Plants Live in Communities
C．How Herbicides May Affect Farm Land
D．Weed Control by Means of Herbicides
单项选择题
The composition of a plant community ______.
A．depends on climate and soil type in a virgin environment undamaged by human beings
B．was greatly affected by human beings before they started using chemical substances on the soil
C．was radically transformed by uncivilized human beings
D．refers to plants, trees, climate, type of soil and the ecological environment
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