Mass density distribution of a lamina with equation $x^2+y^{2}25$ and $y0($upper half of a disk$)$ is given by $(x,y)=16.$

$($A$)(0\mathrm{.}75$ points$)$ Find the mass of the lamina.

$($B$)(2\mathrm{.}5$ points$)$ Find the center of gravity of the lamina.

$($C$)(0\mathrm{.}25$ points$)$ Does the answer in part $($B$)$ make sense?