To find the average rate of change in height for the balloon over the time period, you can use the formula:
\[ \text{Average Rate of Change} = \frac{\text{Change in Height}}{\text{Change in Time}} \]
First, calculate the change in height:
\[ \text{Change in Height} = \text{Final Height} - \text{Initial Height} = 10,500 \text{ ft} - 1,200 \text{ ft} = 9,300 \text{ ft} \]
Next, calculate the change in time:
\[ \text{Change in Time} = \text{Final Time} - \text{Initial Time} = 7 \text{ min} - 2 \text{ min} = 5 \text{ min} \]
Now, plug these values into the average rate of change formula:
\[ \text{Average Rate of Change} = \frac{9,300 \text{ ft}}{5 \text{ min}} = 1,860 \text{ ft/min} \]
Thus, the average rate of change in height for the balloon over this period of time is 1,860 ft/min.
So, the correct response is:
1,860 ft./min.