When Cons $H_2SO_4$ was added into an unknown salt present in a test tube, a brown gas (A) was evolved . This gas intensified when copper turnings were also added into this test tube . On cooling , the gas (A) changed into a colourless gas (B) (a) identify the gases A and B.

Question

$\begin{array}{1 1} (a)\;A = CO_2(g) , B = S_2O_3(g) \\ (b)\;A = NO_2(g) , B = S_2O_4(g) \\ (c)\;A = NO_2(g) , B = N_2O_4(g)\\(d)\;A = NO(g) , B = N_2O_4(g) \end{array}$