Most IP layer-based proxy mechanisms，such as network address translation (NAT)，only supportuni-directional proxy，from the internal network to external network(the Interned．The proxvestablishes the communication channel by (71) IP addresses．The IP addresses must be (72) ．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 (73) for each application that uses (74) channels． SOCKS identifies communication targets through domain names，overcoming the restriction of Usina private IP addresses．SOCKS can use domain names to establish communication between separate LANs，with (75) IP addresses．

A．duplicating

B．crossing

C．overlapping

D．repeating

当前在我国造成环境污染的主要原因是（）

A 、汽车废气的排放

B 、自然灾害的污染

C 、工业“三废”未经处理的排放

D 、大量使用农药和化肥

E 、生活“三废”未经处理的排放