An ideal-gas reaction equilibrium is represented as $$A + B \rightleftharpoons C + D$$. Considering the fact that all components are ideal and that only A and B are present initially, which of the following options is ALWAYS true at equilibrium, where $$N$$ is the number of moles of the species

A

$$N_C = N_A$$

B

$$N_C + N_D = N_A + N_B$$

C

$$N_C = N_D$$

D

$$N_A = N_B$$