The equation $\large\frac{x^2}{5-a}+\frac{y^2}{4-a}$$=1$ represent ellipse if value of 'a' belongs to what value?

Question

$\begin{array}{1 1}(a)\;a \in (-\infty,5)\\(b)\;a \in (-\infty,7)\\(c)\;a \in (-\infty,4)\\(d)\;\text{None of these}\end{array}$