If $\; (X =4^{n} -3n-1) : n \in N \;$ and $\;Y=9(n-1) : n \in N\;$ where N is the set of natural numbers then $\;X \cup Y\;$ is equal to :

Question

$(a)\;X\qquad(b)\;Y\qquad(c)\;N\qquad(d)\;Y-X$