If $\frac{{\cos }^2\theta }{a}=\frac{{\sin }^2\theta }{b}$ then $\frac{{\cos }^4\theta }{a}+\frac{{\sin }^4\theta }{b}=$

• A

  $\frac{1}{a+b}$

• B

  $\frac{1}{{(a+b)}^2}$

• C

  $\frac{1}{a^2}+\frac{1}{b^2}$

• D

  $a+b$