单选题某商品销售量对于进货量的百分比与销售价格成反比例。已知销售单位为9元时，可售出进货量的80%，又知销售价格与进货价格成正比例。已知进货价格为6元时，销售价格为9元。在以上比例系数不变的情况下，当进货价格为8元时，可售出进货量的百分比为______。

单选题The swine flu will probably return in force earlier than seasonal flu usually begins, federal health officials predicted Friday, saying they expected it to erupt as soon as schools open rather than in October or November. The swine flu is still circulating in the United States, especially in summer camps, even though hot weather has arrived and the regular flu season ended months ago, "so we expect challenges when people return to school, when kids are congregating together," Dr. Anne Schuchat, director of respiratory diseases at the Centers for Disease Control and Prevention, said in a telephone news conference held jointly with vaccine experts from the Food and Drug Administration and the Department of Health and Human Services. It is still unclear how many doses of a swine flu vaccine will be available by then, and officials have been reluctant to make firm predictions beyond saying that they expect tens of millions, rather than hundreds of millions, and they plan to distribute them to people who are the most vulnerable, like pregnant women and people who are the most likely to encounter the flu, like health care workers. The number of doses available will depend on how fast seed strains grow, how much protection a small dose provides, and whether immune-system boosters called adjuvants are needed and prove to be safe; adjuvants are not used in American flu vaccines now. Clinical trials testing those questions are expected to take another couple of months, said Dr. Jesse L. Goodman, director of the F. D．A．'s Center for Biologics Evaluation and Research. Assuming a swine flu vaccination campaign begins, it will be voluntary, Dr. Schuchat emphasized, but she "strongly encouraged" pregnant women to get both a seasonal flu shot and a swine flu shot when they are available. The C．D．C．has been closely following the disease in the Southern Hemisphere winter, and it is mimicking the patterns seen in the United States and Mexico in the spring, she said. Most infections and most serious cases are in children and young adults, and those with underlying conditions, including pregnancy, are the most likely to die. Dr. Schuchat likened the spread's unpredictability to that of popcorn: one city could see an explosion of cases and overwhelmed hospitals while another saw few. Her most important message, she added, was that "the virus isn't gone, and we fully expect there will be challenges in the fall. "

单选题A车以110千米/小时的速度由甲地驶往乙地，同时B，C两车分别以90千米/小时和70千米/小时的速度自乙地驶向甲地．途中A车与B车相遇1小时后才与C车相遇，甲、乙两地的距离为( )千米．

单选题设f(x)一阶可导，f(x)＞0，f"(x)＞0，则当Δx＞0时A．B．C．D．

单选题要使在x=0处连续，应补充f(0)等于()

单选题点P 0 (2，3)关于直线x+y=0的对称点是______

单选题设A，B分别是圆周上使得取到最大值和最小值的点，O是坐标原点，则∠AOB的大小为______．A．B．C．D．E．

单选题设函数f(u，v)由关系式f[xg(y)，y]=x+g(y)确定，其中函数g(y)可微，且g(y)≠0，则______．

单选题设f(x)在x=0处满足f'(0)=f"(0)=…=f(n)(0)=0，f(n+1)(0)＞0，则______．

单选题制鞋厂本月计划生产旅游鞋5000双，结果12天就完成了计划的45％，照这样的进度，这个月(按30天计算)旅游鞋的产量将为( )．

单选题袋中有6个球，其中4个红球，2个白球，从中随机取出2个球，则这2个球都为红球的概率为(　　)

单选题购买商品A、B、C，第一次各买2件，共11．4元；第二次购买A商品4件，B商品3件，C商品2件，共14．8元；第三次购买A商品5件，B商品4件，C商品2件，共17．5元，则一件A商品的价格是( )．

单选题 与二次型相对应的实对称矩阵是 A、 B、 C、 D、

单选题的零点个数为()．

单选题微分方程y'+p(x)y=0的通解为()。

单选题某工厂2月份产值比1月份产值增加10%，3月份产值比2月份产值减少10%，那么( )．

单选题In this {{U}}monumental{{/U}} work the entire storehouse of the world's art is surveyed. A. impressive B. expressive C. progressive D. possessive

单选题k个坛子各装n个球，编号为1、2、…、n，从每个坛中各取一个球，所取到的k个球中最大编号是m(1≤m≤n)的概率p是( )．

单选题设其中A可逆．则B-1等于______．

单选题下列函数中，在x=0处可导的是______