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问题 选择题

下列调查活动或实验中，计算所得数值与实际数值相比，可能偏小的是[ ]

A．标志重捕法调查灰喜鹊种群密度时标志物脱落

B．显微镜下计数酵母菌数量时只统计计数室方格内的菌体

C．样方法调查蒲公英种群密度时在分布较密的地区取样

D．调查某遗传病的发病率时以患者家系为调查对象

答案

答案：B

单项选择题

设关系R与关系S具有相同的目，且相对应的属性的值取自同一个域，则R∩S可记作________。

A．{t|t∈R∨t∈S}

B．{t|t∈R∧t┐∈S}("┐∈"表示"不属于")

C．{t|t∈R∧t∈S}

D．{t|t∈R∨t┐∈("┐∈"表示"不属于")
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问答题

"Clean your plate !" and "Be a member of the clean-plate club ! "Just about every kid in the US has heard this from a parent or grandparent. Often, it’s accompanied by an appeal: "Just think about those starving orphans(孤儿) in Africa! "
Sure, we should be grateful for every bite of food. Unfortunately, many people in the US take a few too many bites. Instead of saying "clean the plate", perhaps we should save some food for tomorrow.
According to news reports, US restaurants are partly to blame for the growing bellies (肚子). A waiter puts a plate of food in front of each customer, with two to four times the amount recommended by the government, according to a USA Today story.
Americans traditionally associate quantity with value and most restaurants try to give them that. They serve large portions to stand apart from competitors and to give the customers value. They prefer to have customers complain about too much food rather than too little.
Barbara Rolls, a nutrition professor at Pennsylvania State University, told USA Today that restaurant portion sizes began to grow in the 1970s, the same time that the American waistline began to expand.
Health experts have tried to get many restaurants to serve smaller portions. Now, apparently, some customers are calling for this too. A restaurant industry trade magazine reported last month that 57 percent of more than 4,000 people surveyed believed restaurants serve portions that are too large; 23 percent had no opinion; 20 percent disagreed.
But a closer look at the survey indicates that many Americans who can’t afford fine dining still prefer large portions. Seventy percent of those earning at least $ 150,000 per year prefer smaller portions. But only 45 percent of those earning less than $ 25,000 want smaller.
It’s not that working class Americans don’t want to eat healthy. It’s just that after long hours at low-paying jobs, getting less on their plate hardly seems like a good deal. They live from paycheck to paycheck, happy to save a little money for next year’s Christmas presents.
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