彻底去除牙石是因为牙石（）
A.有机械刺激作用
B.能吸附毒性物质
C.是细菌滞留堆积的部位
D.ABC
E.可引起牙龈增生
参考答案：D
_______ his being late again , he lost his job and had to stay at home.
A．Owe to
B．As result of
C．In case of
D．Because of
下列哪一种推焦串序车辆的利用率高（）。
A、2-1
B、5-2
C、9-2
