Answer: (21.75 / 2) * 2 * pi = 68.42...in, answer D.
Explanation: one revolution from a wheel will be a 360 degree rotation, meaning that if you had laid your finger on the outside of the wheel as it made its full rotation, your finger would have touched the entire circumference of the wheel. Thus, we can represent the length of 1 revolution as the circumference. Thus we take the diameter, and divide it by 2 to get the radius. Then we simply multiply by 2 * pi * r (r = radius). This is the formula for the circumference of a circle.