a2-4b2a2+2ab=(a+2b)(a-2b)a(a+2b)=a-2ba．
故选B．
（）是指泵送钻井液通过地面高压管汇、水龙带、方钻杆、井下钻柱、钻头喷嘴，经过环形空间上返到地面循环系统过程中，因摩擦所引起的压力损失，其数值上等于钻井液循环泵压。
A、关井立压
B、循环压耗
C、环空压耗
D、关井套压
Many people are surprised when they find the Internet(因特网)was set up in the 1960s. At that time, computers were large and expensive. Computer networks(网络) didn’t work well. If one computer in the network broke down, then the whole network stopped. So a new network system (系统) had to be set up. It should be good enough to be used by many different computers. If a part of the network was not working, information could be sent through another part. In this way computer network system would keep on working all the time. At first the Internet was only used by the government, but in the early 1970s,universities(大学) ,hospitals and banks were allowed(允许) to use it too. However, computers were still very expensive and the Internet was difficult to use. By the start of the 1990s, computers became cheaper and easier to use. Today it is easy to get on-line(上网) and it is said that millions of people use the Internet every day. Sending e-mail is more and more popular among students. The Internet has now become one of the most important parts of people’s life.
The Internet has a history of ______ years.
A. less than 30 B. about 40 C. about 20
