单选题：混凝土采用覆盖浇水养护的时间的要求中，对掺用缓凝型外加剂、矿物掺合料或有抗渗性要求的混凝土，不得少于（　　）d。

题目内容：
混凝土采用覆盖浇水养护的时间的要求中，对掺用缓凝型外加剂、矿物掺合料或有抗渗性要求的混凝土，不得少于（　　）d。 A．7
B．14
C．21
D．30

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答案解析：

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