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六选二
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NO.258
中等
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本题平均耗时:2分10秒
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正确率:76%
Select the two answer choices that, when used to complete the sentence, fit the meaning of the sentence as a whole and produce completed sentences that are alike in meaning.

Any antimatter in our part of the universe is necessarily _____ because of the overwhelming preponderance of ordinary matter, by which antimatter is quickly annihilated. 

short-lived
nebulous
scarce
concrete
substantial
ephemeral

Select two answer choices.

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