井下电焊、气焊和喷灯焊接等工作地点的风流中，瓦斯浓度不得超过（）
A.0.75%
B.1.5%
C.1.0%
D.0.5%
参考答案：D
I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased．
A．while; when
B．when; what
C．before; that
D．when; that
越王勾践剑剑把底部有多少个同心圆（）
A、11.0
B、12.0
C、13.0
D、14.0
