D They did their best to learn how to use the machines.

　　(E)

　　Wind is the great maker of waves. There are exceptions, such as the tidal (潮汐的) waves sometimes produced by earthquakes under the sea. But the waves most of us know are produced by winds blowing over the sea .

　　Now before constructing an imaginary life history of a typical wave, we need to know certain

　　physical things about it. A wave has height, from trough ( low point) to crest ( high Point) . It has lengh-the distance from this crest to that of the following wave. The period of the wave means the time it takes for succeeding crests to pass a fixed point. None of these things stays the same--for all depend upon the wind, upon the depth of the water and many other matters.

　　The water that makes up a wave does not advance with it across the sea. Each drop of water turns around in a little circle with the passing of the wave, but returns very nearly to its original position. And it is fortunate that this is so. For if the huge masses of water that make up a wave actually moved across the sea, sailing would be impossible.

　　If we want to find the speed of a wave, we may use the following way :

　　Speed = wavelength × frequency

　　Here, wavelength is the distance between two high points (crests) , frequency means the number of cycles per second .

　　73.What causes waves?

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