主要用于防火墙的VPN系统，与互联网密钥交换IKE有关的框架协议是（）
A.IPSec
B.L2F
C.PPTP
D.GRE
参考答案：A
—My grandmother died last night. I'm very sad about it.
—________. [ ]
A. You' re welcome
B. That's right
C. I 'm very sorry to hear that
D. I hope you 'll feel better soon.
每个生产矿井，必须至少有两个能行人的通到地面的安全出口，各个出口的距离不得小于（）米。
A、20
B、30
C、40
