Express the equilibrium constant for the following reaction.

$4Al(s)+3O_2(g)2A{l}_2{O}_3(s)$

$K=[3O_2{]}^3$

$K=\frac{1}{[O_2{]}^3}$

$K=\frac{1}{[3O_2{]}^3}$

$K=[O_2{]}^3$

$K=\frac{[A{l}_2{O}_3{]}^2}{[O_2{]}^3[Al{]}^4}$