Answer:
12 units per second (Answer C)
Explanation:
The "average rate of change of a function on an interval" is essentially the following ratio:
change in function over the interval
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time interval
Here, the time interval is [1, 3] and the corresponding change in the function is found by subtracting f(1) from f(3):
27 - 3
average rate of change: ------------- = 24/2 = 12 units per second
3 - 1