---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
网友回答
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。