单选题[2015年12月]利用长度为a和b的两种管材能连接成长度为37的管道。(单位：米) (1)a=3，6=5； (2)a=4，b=6。

单选题放满一个水池，如果同时打开1号、2号、3号阀门，则20分钟可以完成；如果同时打开2号、3号、4号阀门，则21分钟可以完成；如果同时打开1号、3号、4阀门，则28分钟可以完成；如果同时打开1号、2号、4号阀门，则30分钟可以完成。那么同时打开1号、2号、3号、4号阀门，需要______分钟可以完成 A.14 B.15 C.16 D.17 E.18

单选题设直线nx+(n+1)y=1(n为正整数)与两坐标轴围成的三角形面积为Sn，n=1，2，…，2009，则S1+S2+…+S2009=______．A．B．C．D．E．以上结论都不正确

单选题 设矩阵的秩为，且,为阶单位阵，则下列结论正确的是 A、的任意个列向量必线性无关， B、的任意的一个阶子式不等于， C、若矩阵满足，则, D、通过初等行变换，必可以化为的形式。

单选题某单位用2160万元购得一块空地，计划在该地块上建造一栋至少10层、每层2000平方米的楼房．经测算，如果将楼房建为x(x≥10)层，则每平方米的平均建筑费用为560+48x(单位：元)．为了使楼房每平方米的平均综合费用最少，该楼房应建为______层．(注：平均综合费用=平均建筑费用+平均购地费用，)

单选题Of all the areas of learning the most important is the development of attitudes. Emotional reactions as well as logical thought processes affect the behavior of most people. "The burnt child fears the fire" is one instance. Another is the rise of despots like Hitler. Both of these examples also point up the fact that attitudes stem from experience. In the one case the experience was direct and impressive, in the other case it was indirect and cumulative. The Nazis were formed largely by the speeches they heard and the books they read. The classroom teachers in the elementary school are in a strategic position to influence attitudes. This is true partly because children acquire attitudes from those adults whose opinion they respect. Another reason is that pupils often delve somewhat deeply into a subject in school that has only been touched upon at home or has possibly never occurred to them before. To a child who had previously acquired little knowledge of Mexico, his teacher's method of handling such a unit would greatly affect his attitude towards Mexicans. The media through which the teacher can develop wholesome attitudes are innumerable. Social attitudes (with special reference to races and nationalities) science matters of health and safety, the very atmosphere of the classroom--these are a few of the fertile fields for the induction of proper emotional reactions. However, when children come to school with undesirable attitudes, it is unwise for the teacher to attempt to change their feelings by cheating or scolding them. She can achieve the proper effect by helping them obtain constructive experiences. Finally, a teacher must constantly evaluate her own attitudes, because her influence can be harmful if she has personal prejudices. This is especially true in respect to controversial issues and questions on which children should be encouraged to reach their own decisions as a result of objective analysis of all the facts.

单选题在一条公路上，汽车A，B，C分别以每小时80km，70km，50km的速度匀速行驶，汽车A从甲站开向乙站，同时车B、车C从乙站出发与车A相向而行开往甲站，途中车A与车B相遇两小时后再与车C相遇，那么甲乙两站相距( )．

单选题设f(x)为连续的偶函数，且F(x)=，则F(-x)等于______．

单选题设则f(x)在[0，+∞)上

单选题平面中的4个点P1，P2，P3，P4在某个球面上，P1P2=P2P3=P3P4=P4P1=3，球心到该平面的距离是其半径的一半，则球的体积是()．

单选题，则行列式|A|的所有元素的代数余子式之和为()

单选题数列{a n }的前n项和为S n ，则S 2n =30．( ) (1)在等差数列{a n }中，S n =10，S 3n =60． (2)在等比数列{a n }中，S n =10，S 3n =70．