I have been collecting stamps I was nine years old.
A. before B. if C. since D. in
I have been collecting stamps I was nine years old.
A. before B. if C. since D. in
C。考查连词的用法。before意为“在……之前”; if意为“如果”; since意为“自从”, 常用于现在完成时中。根据句意: 我自从九岁的时候就开始收集邮票了。故选C。