单选题某中学班主任王老师领一班同学去种树，学生恰好平均分成三组，如果老师与学生每人种树一样多，则共种了572棵且每人种树不超过20棵，且班级学生大于30人，那么这个班有学生______人。 A．32 B．40 C．45 D．51

单选题下列函数中，为减函数的是(　　)

单选题方程z=x 2 +y 2 表示的曲面是

单选题A．π．B．C．D．

单选题曲线的渐近线的条数为______.

单选题设A，B，C是相互独立的随机事件，且0＜P(C)＜1，则下列给出的四对事件中不相互独立的是______．A．B．C．D．

单选题设A为n阶可逆矩阵，λ为A的特征值，则A*的一个特征值为______．A．B．C．λ|A|D．λ|A|n-1

单选题设，则函数f(x)在x=a处____．

单选题设A，B是两个圆(x-2) 2 +(y+2) 2 =3和(x-1) 2 +(y-1) 2 =2的交点，则过A，B两点的直线方程为( )．

单选题设函数f(x)在区间(0，1)内可导，f"(x)＞0，则在(0，1)内f(x)______．

单选题f(x)在(-∞，+∞)上连续，且f(x)＞0，则的单调性为()。

单选题若圆的方程是x2+y2=1，则它的右半圆(在第一象限和第四象限内的部分)的方程是______．A．B．C．D．E．

单选题曲线x2-2x+y2-3=0到直线3x+4y+5=0的最长距离是______。A．B．4C．5D．E．6

单选题半圆形闸门半径为R(米)，将其垂直放入水中，且直径与水面齐，设水密度ρ=1．若坐标原点取在圆心，x轴正向朝下，则闸门所受压力P为______A．．B．．C．．D．．

单选题一所四年制大学每年的毕业生七月份离校，新生九月份入学，该校2001年招生2000名，之后每年比上一年多招200名，则该校2007年九月底的在校学生有______．

单选题直角边之和为12的直角三角形面积最大值等于______．

单选题车间共有40人，某次技术操作考核的平均成绩为80分，其中男工平均成绩为83分，女工平均成绩为78分．该车间有女工( )． A．16人 B．18人 C．20人 D．24人 E．28人

单选题已知，那么=______。A．B．C．D．

单选题Mark Twain once observed that giving up smoking is easy. He knew, because he' d done it hundreds of times himself. Giving up for ever is a trifle more difficult, apparently, and it is well known that it is much more difficult for some people than for others. Why is this so? Few doctors believe any longer that it is simply a question of will power. And for those people that continue to view addicts as merely "weak", recent genetic research may force a rethink. A study conducted by Jacqueline Vink, of the Free University of Amsterdam, used a database called the Netherlands Twin Register to analyze the smoking habits of twins. Her results, published in the Pharmacogenomics Journal, suggest that an individual's degree of nicotine dependence, and even the number of cigarettes he smokes per day, are strongly genetically influenced. The Netherlands Twin Register is a voluntary database that contains details of some 7,000 pairs of adult twins (aged between 15 and 70) and 28,000 pairs of childhood twins. Such databases are prized by geneticists because they allow the comparison of identical twins (who share all their genes) with fraternal twins (who share half). In this case, however, Dr. Vink did not make use of that fact. For her, the database was merely a convenient repository of information. Instead of comparing identical and fraternal twins, she concentrated on the adult fraternal twins, most of whom had completed questionnaires about their habits, including smoking, and 536 of whom had given DNA samples to the register. The human genome is huge. It consists of billions of DNA "letters", some of which can be strung together to make sense (the genes) but many of which have either no function, or an unknown function. To follow what is going on, geneticists rely on markers they have identified within the genome. These are places where the genetic letters may vary between individuals. If a particular variant is routinely associated with a particular physical feature or a behavior pattern, it suggests that a particular version of a nearby gene is influencing that feature or behavior. Dr. Vink found four markers which seemed to be associated with smoking. They were on chromosomes 3, 0, 10 and 14, suggesting that at least four genes are involved. Dr. Vink hopes that finding genes responsible for nicotine dependence will make it possible to identify the causes of such dependence. That will help to classify smokers better (some are social smokers while others are physically addicted) and thus enable "quitting" programs to be customized. Results such as Dr. Vink's must be interpreted with care. Association studies, as such projects are known, have a disturbing habit of disappearing, as it were, in a puff of smoke when someone tries to replicate them. But if Dr. Vink really has exposed a genetic link with addiction, then Mark Twain's problem may eventually become a thing of the past.

单选题设f(x)=(1+x)ex，则f(x)( ) A．有极小值 B．有极大值 C．无极值 D．是否有极值不能确定