اگر $A = \left\{ {3,4,5,6,7} \right\}$ و $B = \left\{ {x \in N\left| {4 \lt x \lt 9} \right.} \right\}$ باشد $A \cup B$ کدام گزینه است؟
1 )
$\left\{ {5,6,7} \right\}$
$\left\{ {3,4,5,6,7,8} \right\}$
3 )
$\left\{ {3,4} \right\}$
4 )
$\left\{ {3,5,6,7,8} \right\}$