4. Most people think it&aposs fine to be “busy as a beaver.” Little do they know. Beavers may work hard, but often they don&apost get much done.

Beavers are supposed to be great tree cutters. It is true that a beaver can gnaw through a tree very quickly. (A six-inch birch takes about ten minutes.) But then what? Often the beaver does not make use of the tree. One expert says that beavers waste one out of every five trees they cut.

For one thing, they do not choose their trees wisely. One bunch of beavers cut down a cottonwood tree more than one hundred feet tall. Then they found that they could not move it.

In thick woods a tree sometimes won&apost fall down. It gets stuck in the other trees. Of course, doesn&apost think to cut down the trees that are in the way. So a good tree goes to waste.

Some people think that beavers can make a tree fall the way they want it to. Not true. (In fact, a beaver sometimes gets pinned under a falling tree.) When beavers cut a tree near a stream, it usually falls into the water. But they do not plan it that way. The fact is that most trees lean toward the water to start with.

Now what about dam building? Most beaver dams are wonders of engineering. The best ones are strongly built of trees, stones, and mud. They are wide at the bottom and narrow at the top.

Beavers think nothing of building a dam more than two hundred feet long. One dam, in Montana, was more than two thousand feet long. The largest one ever seen was in New Hampshire. It stretched four thousand feet. It made a lake large enough to hold forty beaver homes.

So beavers do build good dams. But they don&apost always build them in the right places. They just don&apost plan. They will build a dam across the widest part of the stream. They don&apost try to find a place where the stream is narrow. So a lot of their hard work is wasted.

Beavers should learn that it&aposs not enough to be busy. You have to know what you&aposre doing, too. For example, there was one Oregon beaver that really was a worker. It decided to fix a leak in a man-made dam. After five days of work it gave up. The leak it was trying to block was the lock that boats go through.

What is the main idea of this passage?

A. Beavers may be hard working animals, but they don&apost always choose the most ficient mechanisms. B. Beavers are excellent dam builders. C. New Hampshire was the site of the largest beaver dam. D. Beavers are well developed tree cutters. E. Beavers are poor surveyors of aquatic environments in some cases.

答案:A

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