Question

--- ________ did you start to play football?

--- I started to play football when I was six.

1. A.
   When
2. B.
   How long
3. C.
   What time
4. D.
   How far

Answer 1

A
对话中的答语由时间状语从句when I was six来表示，所以选特殊疑问词when。