At $550 K,$ the $K_c$ for the following reaction is $10^{4}\;mol^{-1}\;L$

Question

$X(g) +Y(g) \rightleftharpoons Z(g)$

At equilibrium, it was observed that :

$[X]=\large\frac{1}{2}$$[Y]=\large\frac{1}{2}$$[Z].$

What is the value of $[Z]$ (in mol $L^{-1}$)  at equilibrium ?

$$\begin {array} {1 1} (a)\;2 \times 10^{-4} & \quad (b)\;10^{-4} \\ (c)\;2 \times 10^{4} & \quad (d)\;10^{4} \end {array}$$