下列粉尘中，致癌作用最强的是()
A．石棉尘
B．青石棉
C．滑石粉
D．赤铁矿粉尘
E．镍矿粉尘
参考答案：B
Trees and flowers ________every year to make our country more beautiful.
A．is planted
B．was planted
C．are planted
D．were planted
（）内有气管隆嵴。
A.气管
B.气管杈
C.左主支气管
D.右主支气管
E.肺段支气管
