What were you doing ______ Zhang Fan came in? [ ]
A. when
B. while
C. where
答案:A
混凝土路面振动时,振动棒在每一处的持续时间,不宜少于( )。
A.10s
B.20s
C.30s
D.40s
与粉尘在呼吸道阻留的部位有密切关系的是()
A.粉尘的化学组成
B.粉尘的比重、形状及硬度
C.粉尘的分散度
D.粉尘的溶解
E.粉尘的电荷性