A、Because it is made from rocks on earth. B、Because it is trapped deep down in the ground. C、Because it flows from place to plac

Most of us use oil in some way, and it is difficult to imagine the modern world without oil. But oil is not easy to find and get out of the earth, as it is trapped deep down in the ground. We must first study the rocks carefully. When we think that the rocks in a certain place may contain oil, we will build a metal tower in the location either on the land or on the sea. This metal tower is called a derrick. The derrick is equipped with oil drilling machinery. The machine will cut a narrow hole down into the ground. Meanwhile, a steel pipe is pushed down to stop the sides from falling, and to keep out water. At last, if we have judged correctly, the oil would rush up the pipe with great force. This is basically how an oil well is set up.
    Oil, as we can see, is obtained more easily than coal. For coal, very often we must dig it from a mine and have it sent up to the ground surface. But for oil, it rushes up a pipe. If an oil well is made near the middle of the oil-field, then natural gas can be obtained. In parts of the world such gas is often sent through pipes to distant towns and cities, so that it can be used, like coal-gas, in houses and factories.
Question No. 15 Why is it difficult to find oil?
Question No. 16 What is a derrick?
Question No. 17 Why does the speaker say that oil is obtained more easily than coal?
Question No. 18 According to the speaker, what can be obtained when an oil well is made near the middle of the oil-field?