Passage 3

　　The beavers of North America build a dam to create an artificial lake in which to construct an island home or lodge.Often the beavers work in colonies.After choosing a narrow place in a shallow stream with a firm bottom,they set to work felling trees by standing on their hind legs and gnawing(咬)round the trunks with their large chisel-like teeth.When the tree is down,the beavers lop(砍)off the branches and cut the trunk into suitable lengths which they drag into the stream and sink across the current.Sticks,stones and mud are used to keep the dam in position and make it watertight.

　　In the middle of the lake thus created the beavers use the same materials to build their lodge.When completed this is a dome-shaped(圓屋頂?shù)?，ventilated(通風(fēng)的)structure about 8 feet in diameter and rising well clear of the surface of the lake.There are two entrances,both under water.One of these is used for general purpose,the other as an escape route in an emergency or for bringing in food.

　　The lodge serves as a home,a nursery for the baby beavers and a storehouse for food in winter.The beavers feed chiefly on the bark of trees,of which they keep a plentiful supply at the bottom of the dam,on the ded of the lake and built into the fabric of their home.

　　In winter,when the lake is frozen over and snow covers the ground,the lodge is virtually an impregnable(牢不可破的)fortress(堡壘)which the beavers can leave and enter by swimming under the ice.Beavers are experts at keeping the water in the lake at the right level by constructing canals.They work industriously to maintain both dam and lodge -hence the origin of that popular phrase about being “as busy as a beaver”。

　　8)The main idea of this passage is________.

　　A.the beaver's living habit

　　B.the beaver's eating habit

　　C.the beaver's working habit

　　D.the beaver's protection habit from its enemy

　　9)According to the passage,which of the following is NOT true?

　　A.Beavers have large teeth like shisels.

　　B.Beavers'home is dome-shaped and ventilated.

　　C.The island home has two entrances,both of which are near the surface of water.

　　D.Beavers chiefly eat barks of trees.

　　10)The author adds the popular phrase “as busy a beaver”to the end of the passage just for________.

　　A.introfucing the origin of the popular phrase

　　B.concluding the hard-working character of the beavers

　　C.giving entertainment to the readers

　　D.showing the wide range of knowledge of the author himself