A circular crop field has an irrigation system in which a pipe, with one end attached to a tower at the center of the field, turns continuously to deliver water. The horizontal distance, in meters, from the tip of the pipe to the farmhouse is represented by y = Equals 400 sine (StartFraction pi Over 8 EndFraction x + pi) + 500, where x represents time in hours. What does the amplitude of this function represent?

A. the length of the pipe
B. the distance between the tower and the farmhouse
C. the time it takes for the pipe to complete one full circle
D. the difference between the maximum and minimum distances from the tip of the pipe to the farmhouse