题目内容：

    The salmon(大马哈鱼) is a very strange fish. It is born in a river. But the young salmon takes two years to swim down the rivers towards the Pacific Ocean. For a1. two years, it lives in the sea and becomes fully grown. Then it returns to the river where it was b2.

A grown-up salmon w3. about 25 pounds and is nearly three feet long. The female salmon will not lay the eggs in the Pacific Ocean. When it is almost time to lay them, both male and female salmon begin a great journey.

Each salmon is heading for the river where it was born. A salmon may be a thousand miles from where its life began, yet it is able to r4. to that place. As the salmon swim, larger fish attack them. The salmon are food for some other fish. Some are c5.by fishermen.

The salmon that are left k6. swimming. After they enter the rivers, they head upstream(向上游). If rocks and waterfalls are i7. their way, the salmon leap(跳) over them. A salmon can leap up a fall as high as ten feet. It leaps by swimming at full speed under the water and then r8. to the surface(表面). Almost the whole body of a salmon is made of muscle. It is these powerful muscles that make the salmon f9. for fast swimming, turning quickly and leaping high out of the waterSome reach their river in summer; others in early autumn. They have come hundreds, even thousands of miles and it has t10. them weeks and even months