---How long have you known the doctor ?  

---_________he helped me three years ago.

A. When　           B. Before　          C. Because             D. Since

—How long have you known the doctor ?

—          he helped me three years ago.

A. When　        B. Before　         C. Because      D. Since

—How long have you known the doctor ?

—          he helped me three years ago.

A. When　        B. Before　         C. Because      D. Since