井下充电室风流中以及局部积聚处的氢气浓度不得超过()。
参考答案:0.5%
细胞内主要阴离子是()
A.蛋白质和有机磷酸盐
B.HCO-3
C.SO2_4
D.Cl-
E.OH-
—When did Mr. Smith come here?
—________ nine o'clock yesterday morning. [ ]
A. At
B. For
C. On
D. With