user
My first is a part of the day,   My last a conductor of light, My whole to take measure of time, Is useful by day and by night.  What am I?
A: minute
B: time unit
C: hourglass
D: minutes
E: time period
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>

C

<|end_of_solution|>