2 mole each of A and B were taken in a container and the following reaction took place, 

$2{\mathrm{A}}_{\left(\mathrm{g}\right)}+{\mathrm{B}}_{\left(\mathrm{g}\right)}\rightleftharpoons 2{\mathrm{C}}_{\left(\mathrm{g}\right)}+2{\mathrm{D}}_{\left(\mathrm{g}\right)}$

When the system attained equilibrium:

A is limiting reagent and it's maximum dissociation is < initial mole 2

For any dissociation of A at equillibrium [A] < [B] as reacting in 2:1 rato

For any dissociation of at equillibrium [C] = [D] as they are forming in 1:1 ratio