user
i have one voice.  when im young i walk n 4 when im older i walk on 2 when im young i walk on 3.  im slowest on 3 what am I?
A: best
B: orphan
C: human
D: senior
E: can walk
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>

C

<|end_of_solution|>