单选题甲、乙两车分别从A，B两地同时相向开出，甲车的速度是50km/h，乙车的速度是40km/h．当甲车驶到A，B两地路程的，再前行50km时与乙车相遇．A，B两地的路程是______km．

单选题a，b，c是不完全相等的任意实数，若x=a 2 -bc，y=b 2 -ac，z=c 2 -ab，则x，y，z

单选题The average (arithmetic mean) of five number is 26. After one of the numbers is removed, the average (arithmetic mean) of the remaining numbers is 25. What number has been removed?

单选题甲、乙两人同时从山脚开始爬山，到达山顶后就立即下山．他们两人下山的速度都是各自上山速度的2倍．甲到山顶时，乙距山顶还有400米；甲回到山脚时，乙刚好回到半山腰．则从山脚到山顶的距离为______

单选题A．．B．C．D．

单选题[2009年10月]a+b+c+d+e的最大值是133。 (1)a、b、c、d、e是大于1的自然数，且abcde=2 700； (2)a、b、c、d、e是大于1的自然数，且abcde=2 000。

单选题如果，则的值为()．

单选题已知F1，F2是椭圆的两焦点，过点F2的直线交椭圆于点A，B，若|AB|=5，则|AF1|+|BF1|=______。A．11B．8C．13D．16

单选题设α1，α2，α3，α4线性无关，则{{U}} {{/U}}线性无关． A. α1+α2，α2+α3，α3+α4，α4+α1 B. α1+α2，α2+α3，α3+α4，α3-α4 C. α1，α1+α2，α1+α2+α3，α1+α2+α3+α4，α4+α1 D. α1-α2，α2-α3，α3-α4，α4-α1

单选题设a n =一n 2 +12n+13，则数列的前n项和S n 最大值时n的值是( )．

单选题一家商店为回收资金把甲乙两件商品均以480元一件卖出。已知甲商品赚了20%，乙商品亏了20%，则商店亏盈结果为______

单选题In a science-fiction movie called "Species", a mysterious signal from outer space turns out to describe the genome of an unknown organism. When the inevitable mad scientist synthesizes the DNA described by the instructions, the creature he breeds from it turns out to resemble Natasha Henstridge, an athletic actress. Unfortunately, the alien harbors within her delicate form the destructive powers of a Panzer division, and it all ends badly for the rash geneticist and his laboratory. Glen Evans, chief executive of Egea Biosciences in San Diego, California, acknowledges regretfully that despite seeking his expert opinion--in return for which he was presented with the poster of the striking Mr Henstridge that hangs on his office wall--the producers of "Species" did not hew very closely to his suggestions about the feasibility of their script ideas. Still, they had come to the right man. Dr Evans believes that his firm will soon be able to create, if not an alien succubus, at least a tiny biological machine made of artificial proteins that could mimic the behavior of a living cell. Making such proteins will require the ability to synthesize long stretches of DNA. Existing technology for synthesizing DNA can manage to make genes that encode a few dozen amino acids, but this is too short to produce any interesting proteins. Egea's technology, by contrast, would allow biologists to manufacture genes wholesale. The firm's scientists can make genes long enough to encode 6,000 amino acids. They aim to synthesize a gene for 30,000 amino acids within two years. Using a library of the roughly 1,500 possible "motifs" or folds that a protein can adopt, Egea's scientists employ computers to design new proteins that are likely to have desirable shapes and properties. To synthesize the DNA that encodes these proteins, Egea uses a machine it has dubbed the "genewriter". Dr Evans likens this device to a word-processor for DNA, on which you can type in the sequence of letters defining a piece of DNA and get that molecule out. As Egea extends the length of DNA it can synthesize, Dr Evans envisages encoding not just proteins, but entire biochemical pathways, which are teams of proteins that conduct metabolic processes. A collection of such molecules could conceivably function as a miniature machine that would operate in the body and attack disease, just as the body's own defensive cells do. Perhaps Dr Evans and his colleagues ought to get in touch with their friends in Hollywood.

单选题一艘轮船往返航行于甲、乙两码头之间，若船在静水中的速度不变，则当这条河的水流速度增加50%时，往返一次所需的时间比原来将______ A.不变 B.减少半个小时 C.增加 D.减少1个小时 E.无法判断