问题补充：
---How long have you been studying at this school?
--- I have been studying here.
A. since I was thirteen years old
B. when I was thirteen
C. at the age of thirteen
D. after I was thirteen
答案：
(答案→)A. since I was thirteen years old
B. when I was thirteen
C. at the age of thirteen
D. after I was thirteen
解析：how long多久,答语中只有选项A与时间长短有关,故选A,从我13岁起,我就在这儿学习了.
