单项选择题
A.物理层 B.数据链路层 C.网络层 D.高层
A.255.255.255.252 B.255.255.255.248 C.255.255.255.240 D.255.255.255.196
A.损耗小 B.色散小 C.损耗大 D.只允许单一波长光束传输