C-10-Q155mediumsingle_mcqThe formula of oleum is—aH2S2O7\mathrm{H_2S_2O_7}H2S2O7bH2SO3\mathrm{H_2SO_3}H2SO3cH2SO4\mathrm{H_2SO_4}H2SO4dH2S2O8\mathrm{H_2S_2O_8}H2S2O8