读某区域等高线地形图，字母abcd表示经纬度，且符合160°＞a＞b＞c＞d。回答下

若x：y=1：2，且3x+2y=14，则x²-y²=______．

根据《岩土工程勘察规范》，废渣材料加高坝的勘察可按堆积规模垂直坝轴线布设勘探线，其勘探线至少不少于下列（）项。（）

A.6条

B.5条

C.3条

D.2条

北京市公安局同步开通官方博客、微博与播客，构成三位一体的网络公共平台，倾听群众呼声，了解社情民意，打造“平安北京”。这一举措旨在（）。

①便于了解民意，反映民声

②拓宽公民诉求新渠道

③扩大公民政治参与的权利

④强化市公安局的管理职能

A.①②

B.①④

C.②③

D.②④

SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment. A varietion of the (71) protocol called NRM is essentially the same as SDLC. SDLC is (72) protocol like HDLC, Frame Relay or X.25.An SDLC network is made up of (73) that controls all communications, and (74) . It is capable of (75) operation, but almost all practical applications are strictly half duplex.

（71）是（）

A. HDLC

B. Frame Relay

C. X.25

D. X.21