Identify the option having the correct relation between a and b, if it is known that for hydrogen gas

${\mathrm{C}}_{\mathrm{p}}-{\mathrm{C}}_{\mathrm{v}}=\mathrm{a}$ and also for oxygen gas ${\mathrm{C}}_{\mathrm{p}}-{\mathrm{C}}_{\mathrm{v}}=\mathrm{b}$ (${\mathrm{C}}_{\mathrm{p}}\text{ and }{\mathrm{C}}_{\mathrm{v}}$ are molar specific heats at constant pressure and constant volume respectively)