To find the change in altitude of the airplane over a period of 15 seconds, we can use the following formula:
\[ \text{Change in altitude} = \text{rate of change} \times \text{time} \]
Given that the altitude is decreasing at a rate of 42 feet per second, and the time period is 15 seconds, we can calculate:
\[ \text{Change in altitude} = -42 , \text{feet/second} \times 15 , \text{seconds} = -630 , \text{feet} \]
Thus, the change in altitude over 15 seconds is \(-630 , \text{feet}\).
The answer is: A. -630 feet.