来自路峰的问题
secondaryschoolintheUSusuallycoverssevenyears,gradessixtotwelve这句子里cover为什么要加s变covers?还有lighttravelsfasterthansound那个travel为什么要加s?
secondaryschoolintheUSusuallycoverssevenyears,gradessixtotwelve这句子里cover为什么要加s变covers?
还有lighttravelsfasterthansound那个travel为什么要加s?

