钙是人体内最多的无机盐，它是构成骨骼和牙齿的主要成分。
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About half an hour before the engine is required, cooling water at （） must be circulated.
A. more than the working temperature
B. less than the working temperature
C. the working temperature
D. the same as the surrounding temperature
隐身技术是为隐蔽自己和欺骗、迷惑敌方所采取的各种隐真示假的技术。
A. 正确
B. 错误
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