At the same instant that a $0\mathrm{.}50-kg$ ball is dropped from $25$ m above Earth, a second ball, with a mass of $0\mathrm{.}25$ kg $,$ is thrown straight upward from Earth's surface with an initial speed of $15\frac{m}{s}.$ They move along nearby lines and pass without colliding. At the end of $2\mathrm{.}0$ s the acceleration of the center of mass of the tw ${?}^{-}$holl svstem is:

$($a$)0\mathrm{.}3$ g

$($b$)0\mathrm{.}5$ g

$($c$)0\mathrm{.}7$ g

$($d$)0\mathrm{.}8$ g

$($e$)1\mathrm{.}0$ g