答案：D

解析：

The stopping distance is directly proportional to the square of the initial speed of the car. If s represents the initial speed of the car, in miles per hour, and d represents the stopping distance, you have that the stopping distance is a function of s and that function d of s = c(s^2), where c is a constant. Since the car’s stopping distance is 17 feet for an initial speed of 20 miles per hour, you know that 17 = c20^2. Therefore, c = 17/20^2 = 0.0425, and the car's stopping distance for an initial speed of 40 miles per hour is 0.042540^2 = 68 feet.