d!!他是用one 来代替整个前面的helping others is a habit
D. one后面省略了that.这不是非限制性定句从句, one 就是指a habit, 后面是其定语从句.helping others is a habit,a habit that you can learn even at an early age. 这样看起来是不是顺一点
c,what是疑问词,可以连接句子啊……
d one是先行词 关系代词that或which省略