配砂时碾砂时间和顺序是什么?
参考答案:
先将新砂加入,再加旧砂碾压2~3分钟,加粘土混合2~3分钟,最后加入水碾压5~6分钟,打开混砂机闸门,由皮带送入储料斗内备用。
—Did Jack come back early last night?
—Yes.It was not yet eight o'clock ______ he arrived home.[ ]
A. before
B. when
C. that
D. until
煤粉密度在0.3-0.6㎏/m3空气是危险浓度,大于或小于该浓度爆炸的可能性都会减少。