Solve the Problem $6 \mathrm{ft}$ Tall Walks at a Rate Of $6 \mathrm{ft} / \mathrm{s}$

Solve the problem.
-A man $6 \mathrm{ft}$ tall walks at a rate of $6 \mathrm{ft} / \mathrm{s}$ away from a lamppost that is $12 \mathrm{ft}$ high. At what rate is the length of his shadow changing when he is $45 \mathrm{ft}$ away from the lamppost?

A) $1 \mathrm{ft} / \mathrm{s}$
B) $2 \mathrm{ft} / \mathrm{s}$
C) $45 \mathrm{ft} / \mathrm{s}$
D) $6 \mathrm{ft} / \mathrm{s}$