Helping others is a habit you can learn even at an early age.A. itB. that C. whatD. one

问题补充：

Helping others is a habit, you can learn even at an early age.

A. it

B. that

C. what

D. one

答案：

A. it

B. that

C. what

(答案→)D. one

解析：句意：助人为乐是一种习惯，甚至你可以在很小的时候就可以学会它。one=a/an+单数可数名词，句中相当于a habit，故选D