There is a story about an elm tree. It was planted in the first half of the 20th century on a farm. It grew to be a __1__ tree.
The family that owned the farm kept a bull __2__ to the elm. The bull paced around the tree，dragging a heavy iron chain with it，which __3__ a trench in the bark about three feet off ground. The trench __4__ over the years，though for whatever reason，which did not kill the tree.
After some years，the family sold the __5__ and took their bull. They cut the chain，leaving the loop around the tree and one link hanging down. Over the years，the __6__ slowly covered the rusting chain.
Then one year later，an agricultural disaster struck Michigan in the form of a __7__.It left a path of death across vast areas. All of the elms __8__ the road became infected and died. Everyone __9__ that the old elm would be the next.
The present owner of the farm considered doing the __10__ thing: cutting it down and chopping it up into firewood before it __11__.But he simply could not bring himself to do it. It was as if the old tree had become a family friend. So he decided to let __12__ take its course.
__13__，the tree did not die. Year after year it __14__.Nobody could understand why it was that the only elm was still standing in the county!
Scientists came to __15__ the tree. They observed the scar left by the iron chain，which was almost completely __16__ by the bark and badly rusted now. They decided that it was the chain that __17__ the elm's life. They reasoned that the tree must have __18__ so much iron from the rusting chain that it became __19__ to the fungus.
Look at the wound made by the chain. Our __20__ can give us resources we need to survive. They can truly make us strong.
答案解析：1-20 DACAD BBADC BDACB DBCCA
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