Jim, who is 6 ft tall, is walking directly away from a 15 ft lamppost at a rate of 4 ft per sec. What is the rate of change in the length of Jim's shadow when he is 8 ft from the base of the lamppost?

a. 16/3 feet/sec

b. 8/3 feet/sec

c. 16/5 feet/sec

d. 8/5 feet/sec