To find the area of a circle, you can use the formula:
\[ A = \pi r^2 \]
where \( A \) is the area and \( r \) is the radius.
Given that the diameter is 21 miles, you can find the radius by dividing the diameter by 2:
\[ r = \frac{21 \text{ mi}}{2} = 10.5 \text{ mi} \]
Now, use the radius in the area formula:
\[ A = 3.14 \times (10.5 \text{ mi})^2 \]
Calculating \( (10.5 \text{ mi})^2 \):
\[ 10.5^2 = 110.25 \text{ mi}^2 \]
Now, plug that value into the area formula:
\[ A = 3.14 \times 110.25 \text{ mi}^2 \approx 346.19 \text{ mi}^2 \]
So, the area of the circle is approximately:
\[ \boxed{346.19 \text{ mi}^2} \]
The correct response is 346.19 mi².