What is the purpose of the STP PortFast BPDU guard feature？ （）
A. enforce the placement of the root bridge in the network
B. ensure that a port is transitioned to a forwarding state quickly if a BPDU is received
C. enforce the borders of an STP domain
D. ensure that any BPDUs received are forwarded into the STP domai
（)选择硐室施工方法主要要考虑围岩的稳定性，断面大小和服务年限等因素。
A. 正确
B. 错误
整体搬迁钻机时，首先要察看好行驶路线，道路坡度不得大于（）。
水喷雾灭火系统用于其他系统防护时，不应大于()s。
A. 100
B. 750
C. 300
D. 400
微信支付 
支付宝支付