A、How to release oil from seeds. B、How to test the local market for oil. C、Food and oil. D、Soybean and oil. A短文第1句指出，人类使用植物(种子)油

From the beginning of human history, people have used oils from seeds and nuts.[19]Most of the time these oils are used as food, especially in cooking. But sometimes they have other uses. For example, oils are found in paint and in cleaning products, like soap. Oil is separated from seeds by using pressure. A machine called a press is often used. Sometimes it is surprising to learn how much oil the seeds contain. Sesame, cotton and sunflower seeds, for example, all contain at least 50% oil. Soybean is an important seed around the world, but it is only 20% oil. So chemicals are needed to release oil from soybeans. The first step in pressing the oil from seeds is to crush the seeds between two stones. A cloth container or bag is filled with the crushed seeds. Then the bag is hung up. Some of the oil will flow out of the bag and can be collected. But some oil will remain in the crushed seeds inside the bag. The easiest way to get the rest of the oil out is to place heavy rocks on the crushed material. Oil can also be collected with small, hand-operated machines. Small presses are important in areas where electricity or gasoline cannot be used. They are also a good way to test if a local market for oil exists.
19. What do we learn about the use of oil from the passage?
20. How much oil does soybean contain?
21. Which of the following is the best title for the passage?