user
I am only useful  When I am full,  Yet I am always  Full of holes. 
A: sponge
B: rabbits
C: bitstock
D: riddle
E: cakehole
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>

A

<|end_of_solution|>