单项选择题

The word "heyday" from the second paragraph can be best expressed by ______.

A．the best time
B．the worst time
C．the beginning
D．the ending
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单项选择题
In Line 2 of the second paragraph it refers to ______.
A．interstellar dust
B．star birth
C．the infrared
D．the light of hot young stars
单项选择题
We can infer that the word ambiguous in the last paragraph probably means ______.
A．clear
B．uncertain
C．mysterious
D．satisfying
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