怎样预防网络可能出现的问题（）
A.经常检查线路或接口是否有漏洞
B.设置防火墙
C.解决静电问题，增加接地线
D.以上都对
参考答案：D
26. His younger sister is often made to cry by him, but this time she made him ______.
A．crying
B．cry
C．to cry
D．cries
采用多级压缩制冷循环的原因？
