完形填空。阅读下面短文，从短文后各题所给的四个选项(A、B、C和D)中，选出可以填入空白处的最佳选项。
When you concentrate, you focus on one thing and get other thoughts out of your mind. You concentrate on schoolwork, on sports, or on some music which requires serious 21 effort. The point is that you think of only one 22 at a time, do it and leave it behind you.
23 to concentrate is important. Any adult will tell you they 24 have at least a dozen things on their minds. When they must complete a project by a certain day and hour, they concentrate on it and 25 several people into a work team. This 26 a clear focus on the work—and on getting others to do their 27 .
Adults have a(n) 28 life which may include a husband or a wife, children, and all the work of running a household. Supper must be 29 and the broken back staircase must be fixed before someone gets 30 . If a child is sick, he or she must go to the 31 .
