user
I am black when you buy me, I am red when you use me, then am grey when you through me away What am I?
A: put
B: iron
C: mold
D: coals
E: bargain hunter
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>

D

<|end_of_solution|>