One day a teacher told his students a story: There were three dogs chasing a woodchuck  ,which ran into a hole in a tree. Soon from the tree out ran a white rabbit. The rabbit ran away very fast. _ and driven by the three dogs, the rabbit climbed up a tree. But it couldn't stand steadily, so it fell down to the ground. Before the dogs came to realize what had happened, the rabbit ran away quickly again... Finishing the story, the teacher asked, "Are there any problems with the story?" One student said, "Rabbit cannot climb trees." Another said, "It's impossible for three dogs not to catch a rabbit." When the students couldn't find any more problems with the story, the teacher said, "There is another problem. Where did the woodchuck go? The woodchuck was the goal that the dogs were chasing. But because of the rabbit's appearance, the dogs changed their direction, and our thoughts unknowingly changed, too. The woodchuck disappeared in our minds." Such is life. While holding on to our goals, we are sometimes fascinated by surrounding  beauties or interrupted  by minor details, so we forget our goals. The teacher wanted his students to  _  by telling the story.
keep a cool mind when facing difficulties find as many problems as possible often change their goals in their life concentrate on their goals in their life