单项选择题

What does the sentence "The air helps purify the water" illustrate

A．The ecological balance.
B．The water cycle.
C．The earth' s limited resources.
D．The life cycle.
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单项选择题
Based on the information in the passage, what can be inferred about X-ray microscopes in the future
A．They will probably replace electron microscopes altogether.
B．They will eventually he much cheaper to produce than they are now.
C．They will provide information not available from other kinds of microscopes.
D．They will eventually change the illumination range that they now use.
单项选择题
What does the passage mainly discuss
A．The detail seen through a microscope.
B．Sources of illumination for microscopes.
C．A new kind of microscope.
D．Outdated microscopic techniques.
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