A man standing 13 feet from the base of a lamppost casts a shadow 5 feet long. If the man is 6 feet tall and walks away from the lamppost at a speed of 400 feet per minute. at what rate, in feet per minute, will the length of his shadow be changing? a) O b) O c) O 4000 13 1000 13 2000 13 2000 d) O 13 4000 e) O 13