Maximum deviation from ideal gas is expected from

A certain gas takes three times as long to effuse out as helium. Its molecular mass will be

For real gases van der Waals equation is written as

$$\left( {p + {{a{n^2}} \over {{V^2}}}} \right)$$ (V $$-$$ nb) = n RT
where $$a$$ and $$b$$ are van der Waals constants. Two sets of gases are
(I)  O₂, CO₂, H₂ and He
(II)  CH₄. O₂ and H₂

The gases given in set-I in increasing order of b and gases given in set-II in decreasing order of $$a$$, are arranged below. Select the correct order from the following

Equal volumes of two monoatomic gases, A and B at same temperature and pressure are mixed. The ratio of specific heats (C_p/C_v) of the mixture will be