题目

How often one hears children wishing they were grown-ups. And old people wishing they were  46  again. Each age has it pleasure and its pain, and the happiest person is the one who enjoys what each age  47  him without wasting his time in useless  48  . Childhood is a time when there are few responsibilities to make life  49  . If a child has good parents, he is  50 , looked after and loved, whatever he may do. It is  51   that he will ever again in his life be  52  so much without having to do anything in return.  53 , life is always giving new things to the child-thin下列物质中，在一定条件下能发生取代反应和加成反应，但不能使酸性高锰酸钾溶液褪色的是A．乙烷 B．甲烷 C．苯 D．乙烯