For the data below, develop a three$-$month moving average:

$\backslash$table$[[$Month$,$Automobile Battery$],[$Sales$,],[$January$,20],[$February$,21],[$March$,15],[$April$,14],[$May$,13],[$June$,16],[$July$,17],[$August$,18],[$September$,20],[$October$,20],[$November$,21],[$December$,23]]$

What is the MAD $($Mean Absolute Deviation$)$ for the entire forecast?

a$.2\mathrm{.}95$

b$.2\mathrm{.}74$

c$.3\mathrm{.}21$

d$.2\mathrm{.}55$