单选题已知x＞0，y＞0，且2x+3y=6，则______．A．有最大值1B．有最小值1C．有最大值D．无最大、最小值

单选题有红，黄，蓝三种颜色的球各7个，每种颜色的7个球分别标有数字1，2，3，4，5，6，7，从中任取3个标号不同的球，这3个球颜色互不相同且所标数字互不相邻的取法种数为

单选题已知等差数列{a n }满足a 1 +a 2 +…+a 101 =0，则有______．

单选题，则以下结论不正确的是()。

单选题 National Parks have more and more visitors each year. In the last ten years the number of campers using the camp sites has more than doubled. Camping as a family vacation has suddenly become extremely popular in America. It is a cheap way to travel; its simple pleasures are a pleasant change from hectic urban life; and it can be enjoyed by children of all ages. In car trunks or in racks on top of cars, families load a tent, sleeping bags, inflatable mattresses, cooking pans and eating utensils, and an ice chest for storing food. When they arrive at a camping ground they find a cleared space in which to pitch their tent, a fireplace for cooking, and usually a picnic table and benches--water and firewood nearby. By evening they are settled under the stars, the campsite around them dotted with lights from cooking fires and lanterns hung from trees. Vacations are not all in resorts or in the wilderness. Swarms of vacationing Americans visit New York and Washington each year. They visit New York because there is no place in the world like this tremendous, exciting city, the busiest port in the world, with its great steel and glass skyscrapers, its theaters and shops, its beauty of skyline and shoreline, and its thrilling five-cent ferry ride past the Statue of Liberty. They visit Washington because it is the nation's capital, where they can see their government at work, tour the public rooms of their President's home, the White House, and walk along the wide avenues to the art galleries and museums. Here they can see exhibits of the native peoples of their land--the Indians and the Eskimos. They can look at Lindbergh's small, fragile plane in which he crossed the Atlantic Ocean. They can ride the elevator to the top of the Washington Monument, visit Washington's Mount Vernon home, and feel the shiver of national pride as they stand at the foot of the great Lincoln Monument and read the stirring words of his Gettysburg Address. For Americans vacation time ends on Labor Day--the first Monday in September. Labor Day is the day when summer cottages are closed, when families head back to their homes. The highways are jammed with cars. The cars are jammed with families and belongings and treasures of the summer. By the time the drivers are back home they sometimes feel that what they need is a vacation.

单选题一条信息可通过如图的网络线由上(A点)往下向各站点传送。例如，信息到b2点可由经a，的站点送达，也可由经a2的站点送达，共有两条途径传送。则信息由A点到达d3的不同途径共有______。

单选题设f(x)有连续的二阶导数，其导函数y=f'(x)的图像如右图所示，令函数y=f(x)的驻点的个数为p，极值点的个数为q，曲线y=f(x)拐点的个数为r，则(A)p=q=r=3．(B)p=q=r=2．(C)p=3，q=2，r=3．(D)p=3，q=2，r=1．

单选题正圆锥体的侧面积是底面积的2倍，则该圆锥体侧面展开后的扇形所对的圆心角为______。A．πB．C．D．

单选题设n阶矩阵A=(α 1 ，α 2 ，…，α n )，B=(β 1 ，β 2 ，…，β n )，AB=(γ 1 ，γ 2 ，…，γ n )，令向量组(Ⅰ)：α 1 ，α 2 ，…，α n ；(Ⅱ)：β 1 ，β 2 ，…，β n ；(Ⅲ)：γ 1 ，γ 2 ，…，γ n ，若向量组(Ⅲ)线性相关，则______．

单选题方程x4-y4-4x2+4y2=0所表示的曲线是( )． (A) 一个半圆和一个圆 (B) 两条相交直线 (C)两条平行直线和一个圆 (D) 两条相交直线和一个圆 (E) 两个圆

单选题方程z2=x2+y2表示的二次曲面是______ A.椭球面 B.柱面 C.圆锥面 D.抛物面

单选题在平面直角坐标系中，已知两点A(cos110°，sin110°)，B(cos50°，sin50°)，则由坐标原点O到线段AB中点M的距离是______．A．B．C．D．1

单选题若在[0，1]上有f(0)=g(0)=0，f(1)=g(1)=α＞0，且f''(x)＞0，g''(x)＜0，则的大小比较关系是______A．I1≥I2≥I3．B．I3≥I2≥I1．C．I2≥I3≥I1．D．I2≥I1≥I3．