煤尘爆炸威力最强的煤尘浓度为()。
A.350~450g/m3;
B.300~400g/m3;
C.300~450g/m3;
D.100~150g/m3
参考答案:B
One of the rules is that under no circumstances _______to bring food, drink or chewing gum into the school.
A.students are allowed
B.allow students
C.are students allowed
D.students be allowed
磨削薄壁套时,砂轮粒度应粗些,硬度应软些,以减少磨削力与磨削热。