The differences between the forecasted and actual values are given as $0,30,-20,$ and $0$ for four different time periods $($i$.$e$.,$ period $1,2,3,$ and $4).$ What is the MAD? Select the closest answer.

$12\mathrm{.}50$

$2\mathrm{.}50$

$25\mathrm{.}00$

$50\mathrm{.}00$

$-6\mathrm{.}50$