道岔尖轨防跳顶铁与密贴尖轨间隙应为（）mm。
参考答案：2～4
—Can I speak to Mr Brown,please?
—________-I'll just see if he's here.[ ]
A. Ring off
B. Hold up
C. Hang on
D. Ring back
硫元素的化合价为+4的物质是
A．H2S
B．S
C．SO2
D．SO3
