答案是 D. one（这里假定 habit 后面有逗号）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"后半句不用词组 a habit 是...那其余选项为什么不用来替代呢你可以试图填其他选项然后看句子：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，第一和第二句语法上显然是错的。第二句呢，如果 habit 后面无逗号，句子就没错，所以上面我说假定有逗号（如题）。认识英语的习惯表达方式需要语感，语感是从多读多听练出来的。