Passagel
Questions 16 to 20 are based on the following passage:
Scientists have learned a lot about the kinds of food people need. They say that there areseveral kinds of food that people should eat every day, they are: (1) green and yellowvegetables of all kinds; (2) citrus (柑桔) fruits and tomatoes; (3) potatoes and other fruits andvegetables; 94) meat of all kinds, fish and eggs; (5) milk and foods made from milk; (6) breador cereal, rice is also in this kind of food; (7) butter, or something like butter.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 atdifferent times of the day. In some places people eat once or twice a day; in other countriespeople eat three or four times a day. Scientists say that none of the differences is reallyimportant. It doesn't matter whether foods are eaten raw or cooked, cannedor frozen. Itdoesn'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.There are two problems, then, in feeding the large number of people on the earth. The firstis to find some way to feed the world's population so that no one is hungry. The second is tomake sure that people everywhere have the right kinds of food to make them grow to be strongand healthy.
17.It is important for people to eat（ ）.