Pick out the wrong statement.

A. Conversion of SO2 to SO3 in Monsanto-4 pass converter is about 98%

B. The chemical formula of oleum is H2S2O7, which is formed by saturating sulphuric acid with sulphur trioxide

C. Vitriol oil is nothing but technical sulphuric acid

D. Decomposition of sulphuric acid on heating does not start before its boiling

Please do not use chat terms. Example: avoid using "grt" instead of "great".