---How long have you been learning Japanese?-I’ve been learning it .A. when I was 10 ye

问题补充：

---How long have you been learning Japanese?

---I’ve been learning it .

A. when I was 10 years old

B. since 10 years ago

C. for 10 years old

D. in 10 years

答案：

A. when I was 10 years old

(答案→)B. since 10 years ago

C. for 10 years old

D. in 10 years

解析：考察时态，根据前文我已经学了……现在完成时，能够和since +一段时间 ago或for+一段时间连用，故选B。