Twenty years ago, your mother invested $$7,000.$ Today, that investment is worth $$320,714\mathrm{.}11.$ What is the average annual rate of return she earned on this investment?

a

$23\mathrm{.}21$

b

$11\mathrm{.}78$

c

$21\mathrm{.}07$

d

$12\mathrm{.}78$

e

$13\mathrm{.}60$