单选题设，B是4×2的非零矩阵，且AB=O，则

单选题直线与平面6x+15y-10z+31=0的夹角ψ为

单选题已知k＞0，则对于反常积分的敛散情况，判别正确的是()．

单选题某洗衣机生产厂家，为了检测其产品无故障的启动次数，从生产的一批洗衣机中任意抽取了5台，如果测得的每台无故障启动次数分别为11300，11000，10700，10000，9500，那么这批洗衣机的平均无故障启动次数大约为______．

单选题设{an}是公差不为0的等差数列，a1=2且a1，a3，a6成等比数列，则{an}的前n项和Sn=______。A．B．C．D．n2+nE．以上答案均不正确

单选题每只蜘蛛有8条腿，蜻蜓有6条腿和2对翅膀，蝉有6条腿和1对翅膀，现有这3种小虫共18只，共有118条腿和20对翅膀，其中蝉的数量为( )只． (A)5 (B)6 (C)7 (D)8 (E)A、B、C、D都不正确

单选题设f(x，y)为连续函数，二次积分交换积分次序后等于______A．B．C．D．

单选题A．1．B．．C．e．D．e2．

单选题设f(t)为连续函数，a是常数，下述命题正确的是 ( )．

单选题某工厂生产某种定型产品，一月份每件产品销售的利润是出厂价的25%(假设利润等于出厂价减去成本)．若二月份每件产品的出厂价降低10%，成本不变，销售件数比一月份增加80%，那么二月份的销售总利润比一月份的销售总利润增长______．

单选题从甲地到乙地，水路比公路近40千米，上午10:00，一艘轮船从甲地驶往乙地，下午1:00，一辆汽车从甲地开往乙地，最后船、车同时到达乙地．若汽车的速度是每小时40千米，轮船的速度是汽车的，则甲乙两地的公路长为______．

单选题设函数f(x)在区间[-1，1]上连续，则x=0是函数的______．A．跳跃间断点B．可去间断点C．无穷间断点D．振荡间断点

单选题方程x 2 -2006|x|=2007所有实数根的和等于______．

单选题函数f(x)=x sin x

单选题设f'x(x0，y0)，f'y(x0，y0)都存在，则

单选题若对一切正实数x恒成立，则y的取值范围是______．

单选题A．0．B．不存在．C．D．

单选题The marvelous telephone and television network that has now enmeshed the whole world, making all men neighbors, cannot be extended into space. It will never be possible to converse with anyone on another planet. Even with today's radio equipment, the messages will take minutes—sometimes hours—on their journey, because radio and light waves travel at the same limited speed of 186, 000 miles a second. Twenty years from now you will be able to listen to a friend on Mars, but the words you hear will have left his mouth at least three minutes earlier, and your reply will take a corresponding time to reach him. In such circumstances, an exchange of verbal messages is possible—but not a conversation. To a culture which has come to take instantaneous communication for granted, as part of the very structure of civilized life, this "time barrier" may have a profound psychological impact. It will be a perpetual reminder of universal laws and limitations against which not all our technology can ever prevail. For it seems as certain as anything can be that no signal--still less any material object—can ever travel faster than light. The velocity of light is the ultimate speed limit, being part of the very, structure of space and time. Within the narrow confines of the solar system, it will not handicap us too severely. At the worst, these will amount to twenty hours—the time it takes a radio signal to span the orbit of Pluto, the outer-most planet. It is when we move out beyond the confines of the solar system that we come face to face with an altogether new order of cosmic reality. Even today, many otherwise educated men—like those savages who can count to three but lump together all numbers beyond four—cannot grasp the profound distinction between solar and stellar space. The first is the space enclosing our neighboring worlds, the planets; the second is that which embraces those distant suns, the stars, and it is literally millions of times greater. There is no such abrupt change of scale in the terrestrial affairs. Many conservative scientists, appalled by these cosmic gulfs, have denied that they can ever be crossed. Some people never learn ; those who sixty years ago scoffed at the possibility of flight, and ten years ago laughed at the idea of travel to the planets, are now quite sure that the stars will always be beyond our reach. And again they are wrong, for they have failed to grasp the great lesson of our age— that if something is possible in theory, and no fundamental scientific laws oppose its realization, then sooner or later it will be achieved. One day we shall discover a really efficient means of propelling our space vehicles. Every technical device is always developed to its limit and the ultimate speed for spaceships is the velocity of light. They will never reach that goal, but they will get very near it. And then the nearest star will be less than five years voyaging from the earth.

单选题在一个101人参加的聚会上，下列结论正确的是______

单选题设y=2x2-lny，则y"=A．B．C．D．