After 10 hours the temperature is shown on the graph as 30 degrees.
50 degrees - 30 degrees = 20 degrees.
The temperature dropped 20 degrees in 10 hours.
Divide the change by the time:
20 degrees / 10 hours = 2 degrees per hour.
Because the temperature dropped, the change would be negative.
The answer is D. -2 degrees per hour.
