If $\dfrac{cos\alpha }{cos\beta }=a$ and $\dfrac{sin\alpha }{sin\beta }=b,$ then the value of ${{\sin }^{2}}\beta ~$in terms of a and b is:
$\dfrac{{{a}^{2}}+1}{{{a}^{2}}-{{b}^{2}}}$
$\dfrac{{{a}^{2}}-{{b}^{2}}}{{{a}^{2}}+{{b}^{2}}}$
$\dfrac{{{a}^{2}}-1}{{{a}^{2}}-{{b}^{2}}}$
$\dfrac{{{a}^{2}}-1}{{{a}^{2}}+{{b}^{2}}}$