下列运算，正确的是 $($ 　　 $)$

A．

$2x+3y=5\mathit{xy}$

B．

${(x-3)}^2=x^2-9$

C．

${\left(x{y}^2\right)}^2=x^2{y}^4$

D．

$x^6÷x^3=x^2$