正常操作的二元精馏塔，塔内某截面上升气相组成Yn+1和下降液相组成Xn的关系是（）。
A、Yn+1>Xn
B、Yn+1〈Xn
C、Yn+1=Xn
D、不能确定
参考答案：A
下图为某地的等高线地形图，图中最粗线之内的范围图上面积约为6平方厘米，而其实际地表面积约为15000平方米，丙地位于40°N，117°E，读图回答1—2题。
1、这幅图的比例尺为[ ]
A．1：1000
B．1：250000
C．1：5000
D．1：50000
2、若在上图所示山区修建公路，则应遵循的原则是
①尽可能避开滑坡泥石流多发区②在山谷中应尽量避开陡坡
③在陡坡段增加“之”字形弯曲④尽可能多跨越河流[ ]
A．②③④
B．①②③
C．①③④
D．①②③④
Like many other aspects of the computer age, Yahoo began as an idea, (1) into a hobby and lately has (2) into a full-time passion. The two developers of Yahoo, David Filo and Jerry Yang, Ph. D candidates (3) Electrical Engineering at Stanford University, started their guide in April 1994 as a way to keep (4) of their personal interest on the Internet. Before long they (5) that their homebrewed lists were becoming too long and (6) . Gradually they began to spend more and more time on Yahoo.
During 1994, they (7) yahoo into a customized database designed to (8) the needs of the thousands of users (9) began to use the service through the closely (10) Internet community. They developed customized software to help them (11) locate, identify and edit material (12) on the Internet. The name Yahoo is (13) to stand for "Yet Another Hierarchical Officious Orale", but Filo and Yang insist they selected the (14) because they considered themselves yahoos. Yahoo itself first (15) on Yang’s workstation, "akebono", while the search engine was (16) on Filo’s computer, "Konishiki".
In early 1995 Marc Andersen, co-founder of Netscape Communication in Mountain View, California, invited Filo and Yang to move their files (17) to larger computers (18) at Netscape. As a result Stanford’s computer network returned to (19) , and both parties benefited. Today, Yahoo (20) organized information on tens of thousands of computers linked to the web.
20（）
A.attains
B.detains
C.maintains
D.contains
