以下三种水泥中，早期强度最低的是（）。
A.P•S
B.P•P
C.P•F
参考答案：C
药品批准文号的格式为（）
A．国药准字+1位字母+8位数字
B．国药试字+1位字母+8位数字
C．二者都不是
D．二者都是
E．其他
— Tom is never late for work. Why is he absent today?
— Something _____ to him. [ ]
A. must happen
B. should have happened
C. could have happened
D. must have happened
