— Hello! Could I speak to Mr. Liu, please?

— Sorry. He _________ for a month.

A. had been away    B. was left C. has been away

采用普通水泥拌制的混凝土，在10°C的条件下养护7d，测得其150mm×150mm×150mm立方体试件的抗压强度为15MPa。估算该混凝土在此温度下28d的强度为25.7MPa。