【多选题】【消耗次数：1】
血凝抑制试验设立的对照有
①
血清对照
②
病毒对照
③
PBS对照（空白对照）
参考答案：
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相关题目
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以下哪种属于病例对照研究的缺点（ ）
①
不适合暴露比例高的因素
②
存在回忆偏倚
③
可以测定暴露和非暴露组疾病的率
④
研究周期长，失访率高
⑤
不适用于罕见病的研究
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reach
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share
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take
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lead
【单选题】
Hi, is Mary there, please?
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Hold on. I’ll get her
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No, she isn’t here
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Yes, she lives here
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Yes, what do you want
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__________, the knowledge economy has become the major characteristic (特征) of the national economy of most developed countries.
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From time to time
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Once in a while
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For the time being
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Once upon a time
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Oh, sorry to bother you.
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That’s okay
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live, living
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