单项选择题

The word "it" (Line 3, Para. 6) probably refers to ______ .

A．carbon dioxide
B．nitrogen
C．atmosphere
D．coal
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单项选择题
What did people do to solve the problem of rain lack around Chad Lake
A．They moved out of this area.
B．They drew water from Lake Chad.
C．They carried water from other areas.
D．They utilize sea water near this area.
单项选择题
What is this passage mainly about
A．Demonstrating the serious weather condition around Lake Chad.
B．Introducing people's activity around Lake Chad.
C．Analyzing of the factors that cause water decreases in Lake Chad.
D．Introducing scientists' work about water condition in Lake Chad.
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