Chris would like to know how long it will take him to double his money if he can earn a $3\mathrm{.}2\%$ return on his investment. If he uses the Rule of $70$ it is:

$23\mathrm{.}75$ years

$21\mathrm{.}88$ years.

$17\mathrm{.}50$ years.

$18\mathrm{.}75$ years.