单项选择题

What does the passage mainly discuss

A．The production of solar light and heat.
B．The physical and chemical nature of life.
C．The conversion of Hydrogen to helium.
D．Radiation in the X - ray part of the spectrum.

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单项选择题

It can be inferred from the passage that the Sun's light travels

A．through solid objects in space
B．in many different directions
C．more slowly than scientists previously believed
D．further in summer than in winter

单项选择题

Which sentence proves that setting the giant reflector is a delicate operation

A．The first sentence in the last paragraph.
B．The second sentence in the last paragraph.
C．The last sentence in the last paragraph.
D．The last sentence in the third paragrap