问题 填空题

某地区有A、B、C三家养鸡场,养鸡的数量分别是12000、8000、4000只,为了预防禽流感,现用合适的抽样方法从中抽取一个容量为120只的样本检查疫情,则应从A、B、C三家养鸡场分别抽取的个体数为 ______.

答案

个体的总数为:12000+8000+4000=24000,每个个体被抽到的频率为

120
24000
=
1
200

应从A养鸡场抽取的个体数为 12000×

1
200
=60,

应从B养鸡场抽取的个体数为 8000×

1
200
=40,

应从C养鸡场抽取的个体数为  4000×

1
200
=20,

则应从A、B、C三家养鸡场分别抽取的个体数为 60,40,20,

故答案为  60,40,20.

填空题
单项选择题

Imagine eating everything delicious you want with none of the fat. That would be great, wouldn’t it

New "fake fat" products appeared on store shelves in the United States recently, but not everyone is happy about it. Makers of the products, which contain a compound called olestra, say food manufacturers can now eliminate fat from certain foods. Critics, however, say the new compound can rob the body of essential vitamins and nutrients and also cause unpleasant side effects in some people. So it’s up to consumers to decide whether the new fat-free products taste good enough to keep eating.

Chemists discovered olestra in the late 1960s, when they were searching for a fat that could be digested by infants more easily. Instead of finding the desired fat, the researchers created a fat that can’t be digested at all.

Normally, special chemicals in the intestines (肠道) "grab" molecules of regular fat and break them down so they can be used by the body. A molecule of regular fat is made up of three molecules of substances called fatty acids.

The fatty acids are absorbed by the intestines and bring with them the essential vitamins A, D, E and K. When fat molecules are present in the intestines with any of those vitamins, the vitamins attach to the molecules and are carried into the bloodstream.

Olestra, which is made from six to eight molecules of fatty acids, is too large for the intestines to absorb. It just slides through the intestines without being broken down. Manufacturers say it’s that ability to slide unchanged through the intestines that makes olestra so valuable as a fit substitute. It provides consumers with the taste of regular fat without any bad effects on the body. But critics say olestra can prevent vitamins A, D, E and K from being absorbed. It can also prevent the absorption of carotenoids (类胡萝卜素), compounds that may reduce the risk of cancer, heart disease, etc.

Manufacturers are adding vitamins A, D, E and K as well as carotenoids to their products now. Even so, some nutritionists are still concerned that people might eat unlimited amounts of food made with the fat substitute without worrying about how many calories they are consuming.

We learn from the passage that olestra is a substance that ()。

A. contains plenty of nutrients

B. makes foods fat-free while keeping them delicious

C. renders foods calorie-free while retaining their vitamins

D. makes food easily digestible