Most IP layer-based proxy mechanisms, such as network address translation (NAT), only support uni-directional proxy, from the internal network to external network (the Internet). The proxy establishes the communication channel by (66) IP addresses. The IP addresses must be (67) . These proxy mechanisms prevent applications that must establish return data channels from establishing their data channels, such as, multimedia applications. IP layer-based proxy mechanisms need additional software (68) for each application that uses (69) channels. SOCKS identifies communication targets through domain names, overcoming the restriction of using private IP addresses. SOCKS can use domain names to establish communication between separate LANs, with (70) IP addresses.
A．duplicating
B．overlapping
C．crossing
D．repeating
时（T为交流的周期），原线圈中的电流瞬时值为0.03A．变压器的原、副线圈的匝数比为（ ）
：3