完形填空 there was once a bridge over a large river.

完形填空 there was once a bridge over a large river.
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原文
There was once a bridge over a large river. Each day, a train would come and the bridge would be turned sideways across the river, allowing the train to cross it.
A man sat in a small room on one side of the river where he operated (操作) the controls to turn the bridge and lock it into place when a train came.
One evening, when the man saw the train lights, he stepped to the controls and tried to turn the bridge into position, but, to his surprise, he found the locking control did not work. If the bridge was not safe in position, it would move back and forth at the ends, causing the train to jump the track and go crashing into the river.
The man hurried across the bridge to the other side of the river where there was a locking lever (操纵杆) that he could operate by hand. Suddenly, he heard a sound from the direction of his room, “Daddy, where are you?”
His four-year-old son was crossing the bridge to look for him. If he took the boy away from the bridge first, he couldn’t get back to the lever in time. But if he didn’t, the boy would be killed by the train. Either the people on the train or his little son must die…
The train passed safely and no one on the train saw the small broken body that was thrown into the river by the train. Nor did they see the man who was still holding the locking lever.