通读下面的短文，掌握其大意。然后，从每小题的四个选项中选出可填入相应空白处的最佳选项。

　　Living one's life is really like driving on a highway. First, to drive well, the driver must carefully examine the 　21　 of his car, just as he cares for his own health. Once, he goes on the highway, he must 　22　 certain rules for his own safety. Just as he must obey certain rules of society in order to be a decent(体面的)human being. For example, on the highway, he is 　23　 to keep a constant speed which can be compared with his 　24　 life. Neither driving nor living must be taken at too 　25　 a speed nor at too slow a speed. If he drives over 　26　, the police will give him a ticket, just as he will be arrested when he breaks the law. Acting too conservatively(保守地), he will be called old-fashioned 　27　 others as they drive at the proper speed. When he wants to change lanes, he must pay 　28　 to what he is doing and check in every direction. It is sometimes dangerous to change our way of life 　29　 one can be sure of success. He 　30　 not make any unnecessary changing of lanes which may be dangerous. Finally, he must discover where he is now by recognizing some 　31　 appearing now and then. If he takes the wrong way, he must return to the right, as 　32　 as he can. But he may thus lose 　33　 and energy both on the road and in life. On the whole, driving on a highway and living one's life are both hard work, 　34　 if he is careful and serious enough, 　35　 will provide him much pleasure.

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