Final answer:
Without the number of hours specified, the temperature change per hour cannot be calculated. The temperature fell by a total of 6 3/5°F or 6.6°F, and this total change would typically be divided by the number of hours to find the per hour change.
Step-by-step explanation:
The question asks about the change in temperature per hour when the temperature falls from 0°F to 6 3/5°F below 0°F. To calculate the temperature change per hour, we need to know the total temperature change and the number of hours over which the change occurred. Since the question does not specify the number of hours, we cannot provide an exact per hour change. Generally, to find the per hour change, you would divide the total change in temperature by the number of hours. In this case, the total temperature change is 6 3/5°F or 6.6°F. For example, if the temperature change occurred over 2 hours, the temperature change per hour would be 6.6°F / 2 hours = 3.3°F per hour, which is not one of the given options.