单选题
It is certain that ______.
A、
an object had exploded three miles above the forest
B、
the explosion was not caused by a comet
C、
an atomic explosion once happened above the ground
D、
some form of life from other planets once visited the earth then
【正确答案】
A
【答案解析】
[分析] 第六行“Recent studies concluded that…”，可以直接找到答案，选A。
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