For the reversible reaction equilibrium is $N_2(g) + O_2(g) \overset{\overset{k_r}{\rightleftharpoons}}{_{k_f}} 2NO(g),$ $C_0 = Ce^{-2.1 \times 10^{-3} t}$ for the forward reaction and $C'_0 = C'e^{-4.2 \times 10^{-4}} t$ for the backward reaction, hence, if both forward and backward reactions are first order reactions then $K_c$ for the above equilibrium is:

Question

(a) 5.0

(b) 2.0

(c) 0.5

(d) 2