$2.$ If a sample of phosphatic rock contains $40\%$ tricalcium phosphate, what is the percentage of calcium in the sample $($Atomic weights: Ca $=40,$ P $=31,$ O $=16)?$

$-$ A$)24\mathrm{.}6\%$

$-$ B$)20\mathrm{.}7\%$

$-$ C$)18\mathrm{.}2\%$

$-$ D$)16\mathrm{.}3\%$