下列计算正确的是 $($　　 $)$

A． ${\left(a^{2}b\right)}^2=a^2{b}^2$B． $a^6÷a^2=a^3$

C． ${\left(3x{y}^2\right)}^2=6x^2{y}^4$D． ${(-m)}^7÷{(-m)}^2=-m^5$