不利用瓦斯、采用干式抽放瓦斯设备时，抽放瓦斯浓度不得低于（）。
A.20%
B.25%
C.30%
D.40%
参考答案：B
Don’t call him at this time tomorrow, because he _________ a meeting with his teachers.
A．is having
B．is going to have
C．will be having
D．has been having
这个标志是何含义？（）
A．避险车道
B．应急车道
C．路肩
D．急弯道
