To find the rate at which Curtis traveled on the first day, we can use the formula for speed, which is:
\[ \text{Speed} = \frac{\text{Distance}}{\text{Time}} \]
In this case, the distance traveled is 816 miles and the time taken is 12 hours. Plugging in the values:
\[ \text{Speed} = \frac{816 \text{ miles}}{12 \text{ hours}} = 68 \text{ miles per hour} \]
Thus, Curtis traveled at a rate of 68 miles per hour on the first day.
The answer is: B. 68 miles per hour