对有抗渗要求的混凝土，矿渣水泥、火山灰水泥等拌制的混凝土养护时间不得少于( )d。
A．14
B．18
C．21
D．30
参考答案：C
以下属于第三代头孢菌素的是（）
A.头孢噻吩
B.头孢羟氨苄
C.头孢克洛
D.头孢噻肟
E.头孢匹罗
He doesn’t do his homework ________, though he has ________.
A．carefully enough; enough time
B．enough careful; time enough
C．careful enough; enough time
D．enough carefully; time enough
