答案是 D. one（這里假定 habit 后面有逗號）完整的句子是Helping others is a habit , one (that) you can learn even at an early age.that 被省略了句子的 one 相等于,并替代,"a habit"后半句不用詞組 a habit 是...那其余選項為什么不用來替代呢你可以試圖填其他選項然后看句子：Helping others is a habit , (it) you can learn even at an early age.Helping others is a habit , (that) you can learn even at an early age.Helping others is a habit , (what) you can learn even at an early age.你看，第一和第二句語法上顯然是錯的。第二句呢，如果 habit 后面無逗號，句子就沒錯，所以上面我說假定有逗號（如題）。認(rèn)識英語的習(xí)慣表達(dá)方式需要語感，語感是從多讀多聽練出來的。