下列运算中，正确的是（    ）                                                      

A：$a^2+a^2=2a^4$

B：$a^2·a^3=a^6$

C：${\left(-3x\right)}^3÷\left(-3x\right)=9x^2$

D：${\left(-a{b}^2\right)}^2=-a^2{b}^4$