【正确答案】 B

【答案解析】[解析] 句中more是比较级，要与than连用，在这里than在句中作为关系代词引导定语从句，than在从句中可充当主语，例如：Don’t eat more food than is necessary．不要暴饮暴食。There were more tourists than was expected．游客比预计的多。as也可引导限制性定语从句，但一般要和the same或such等连用，而不与比较级more连用，例如：We need such materials as can bear high temperature and pressure．我们需要那些能够耐高温耐高压的材料。Ultrasonic sound is such inaudible as to human ear．超声波是一种人耳听不到的声音。that可引导定语从句，不和more搭配，例如：I’ll give you anything that you ask for．你要什么，我给什么。Mr．Philip is the only person that can complete this task．菲力普先生是唯一能完成这项任务的人。but也可以作关系代词，用来引导定语从句，此时but一般用在否定句中，相当于that not，who not，例如：There is no mother but loves her children．没有不爱自己孩子的母亲。There is not a conductor but resists the flow of the electrical charges．没有一种对电荷的流动没有阻力的导体。