单项选择题

Why did it take so long to develop the X-ray microscope

A．Funds for research were insufficient.
B．The source of illumination was not bright enough until recently.
C．Materials used to manufacture X-ray tubes were difficult to obtain.
D．X-ray microscopes were too complicated to operate.

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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.

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.