作业精灵老师示例：
One day, a duck fell into a hole. He cried for help. A monkey came. He
tried to help the duck with a pole, but that didn't help. An elephant came.
He used his nose to reach the duck. But he failed.A bear came. He had a
good idea. They poured some water into the hole. At last, the duck got
out.They were happy.
