A person has a height 1.8 m. Beginning directly under a streetlight that is 6 m above the ground, they walk away from their starting position at 7 m/s. Find the speed at which the top of the person's shadow moves.
a. 6 m/s.
b. 8 m/s
c. 10 m/s
d. 12 m/s
e. 14 m/s