恐龙大约在六千五百万年前就消失了。科学家试着解释它们消失的原因。
Dinosaurs disappeared about 65 million years ago. Scientists try to explain their disappearance.
没有人确切地知道恐龙是由于什么原因，又是怎样在这么短的时间里从地球上消失的。
Nobody knows for sure ________________ dinosaurs disappeared from the earth in such a short time.
没有人确切地知道恐龙是由于什么原因，又是怎样在这么短的时间里从地球上消失的。
Nobody knows for sure ________________ dinosaurs disappeared from the earth in such a short time.
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