下列计算正确的是 $($ 　　 $)$

A．

$a^2·a^3=a^6$

B．

${(x+y)}^2=x^2+y^2$

C．

${(a^5÷a^2)}^2=a^6$

D．

${(-3\mathit{xy})}^2=9x{y}^2$