阅读理解。
Paragraph 1
Scientists have learned a lot about the kinds of food people need. They say that there are several kinds of
food that people should eat every day. They are: (1) green and yellow vegetables of all kinds. (2) citrus (柑桔)
fruits and tomatoes; (3) potatoes and other fruits and vegetables; (4) meat of all kinds, fish and eggs; (5) milk
and foods made from milk; (6) bread or cereal (谷类), rice is also in this kind of food; (7) butter, or something
like butter.
Paragraph 2
People in different countries and different places of the world eat different kinds of things. Foods are
cooked and eaten in many different kinds of ways. People in different countries eat at different times of the
day. In some places people eat once or twice a day; in other countries people eat three or four times a day.
Scientists say that none of the differences is really important. It doesn't matter whether foods are eaten raw
(生的) or cooked, canned or frozen. It doesn't matter if a person eats dinner at 4 o'clock in the afternoon or
at eleven o'clock at night. The important thing is what you eat every day.
Paragraph 3
There are two problems, then, in feeding the large number of people on earth. The first is to find some
ways to feed the world's population so that no one is hungry.
The second is to make sure that people everywhere have the right kinds of food to make them grow to
be strong and healthy.
1. According to the scientists, which of the following groups of food is the healthiest for your lunch? [ ]
A. chicken, apples, cereal, cabbages
B. potatoes, carrots, rice, bread
C. oranges, bananas, fish, tomatoes
D. beef, pork, fish, milk
2. It is important for people to eat _____. [ ]
A. three times a day
B. dinner at the right time
C. cooked food all the day
D. something from each of the seven kinds of food every day
3. Which of the following is NOT true? [ ]
A. People in some places don't have enough to eat.
B. There are too many people in the world.
C. One of the problems is that no one is hungry.
D. The scientists are trying to make people grow to be strong and healthy.
4. If there is Paragraph 4, what do you think is going to be talked about? [ ]
A. When people should eat
B. What to do with the two problems
C. How to cook food in different ways
D. Why people eat different kinds of food
，物体在水平恒力F1的作用下，在水平面上作匀速直线运动，今若再对物体施加一大小与F1相等的恒力F2后，要使物体仍然能沿原速度方向在水平面上作匀速直线运动，那么力F2的方向应满足什么条件？
（1）当F2为推力且与F1夹角为
时，有
因为F2=F1，解得
(2)当F2为拉力且与水平方向成