【单选题】【消耗次数：1】
预防一些传染病最有效、最经济的措施是（）。
①
定期体检
②
无偿献血
③
锻炼身体
④
接种疫苗
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如果传染病病因不明确、传染来源一时调查不清，且无法获得有效的疫苗或预防性药物时，是传染病预防控制的关键措施：
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