单选题
The harm radiation has done to the Apollo crew members ______.
A、
is insignificant
B、
seems overestimated
C、
is enormous
D、
remains unknown
【正确答案】
D
【答案解析】
[解析] 从第二段中的“So far，no dangerous amounts of radiation have been reposed，but Apollo missions have been quite short”可推知D是正确选项。
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