Helping others is a habit you can learn even at an early age.A. itB. thatC. 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

(答案→)B. that

C. what

D. one

解析：考查定语从句。a habit是先行词，在从句you can learn even at an early age.中作learn的宾语。所以选B。