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【单选题】
It is a key step in understanding the new strain of coronavirus and producing a _____. 这是了解新型病毒并研制疫苗的关键步骤。A.
vaccine
B.
source
C.
period
D.
who
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A.
排痰化痰
B.
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C.
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D.
减少急性加重的程度
E.
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