Final answer:
The temperature decreases by 21°F after three hours if it is falling at a rate of 7°F per hour.
Step-by-step explanation:
If the temperature outside is falling at a rate of 7°F per hour, the change in temperature after three hours can be calculated by multiplying the hourly rate of change by the number of hours. To do this, we take 7°F/hour and multiply it by 3 hours.
7°F/hour × 3 hours = 21°F
Therefore, the correct answer is C. 21°F.