阅读下面的短文，掌握其大意，然后从短文后各题所给的A、B、C、D四个选项中，选择最佳选项。

　　Some boys join the navy(海军)when they are young．They are then given a 　　1　　 of training as sailors．It is a long course, both on land and at sea．During it, the boys study things like mathematics and science 　　2　　 learning to tie the knots(打结), fire guns and to do other practical things．

　　One of the important things they are taught is how to swim．　　3　　 the old days, many sailors were unable to swim, nowadays it is hard to find one that cannot．

　　At one school for sailors, the swimming instructor(教练)was excellent．He had 　　4　　 never met a boy whom he had 　　5　　 to teach to swim by the time the course ended．One year, however, there was one particular(特别的)boy on the training course who seemed quite 　　6　　 to learn to swim．The instructor tried giving him extra(额外的)lessons, but he had no success at all, whatever he did．In the end, 　　7　　 the time drew near for the training to end, he had to admit(承认)failure．

　　One day, he 　　8　　 the boy to meet him after the swimming lesson and said to him, “John, I have tried very hard to teach you to swim, but I have failed for the first time in my life．Now I want to give you a piece of 　　9　　．Listen carefully．”

　　“Yes, sir, ” answered the boy．

　　“Well, ” the instructor went on, “if you are ever in a ship and it will sink(沉没), just jump over the side into the sea, go right down to the bottom and run to the shore(海岸)as 　　10　　 as you can．That is the only way you will save your life．”

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