单选题
The word "period" in Paragraph 3 means the length of time
A、
over which stars vary in brightness.
B、
for which Eta Carinae's brightening lasts.
C、
Eta Carinae takes to become a supernova.
D、
it takes the laser from Eta Carinae to travel to the earth.
【正确答案】
A
【答案解析】
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