In contact process, to which substance are adequate quantities of water added to convert it to sulphuric acid?

Explanation

In the contact process, sulfur dioxide (formed by roasting sulfur in oxygen) is reacted with oxygen to form sulfur trioxide, which is mixed with H2SO4 to form oleum or fuming sulfuric acid, with the formula of H2S2O7. This is then mixed with adequate quantities of water to form aqueous sulfuric acid, so the answer is A.Option B is hydrogen sulfate, the species formed when H2SO4 donates one proton. This is not involved in the contact process.Option C is SO3, which can react with water to directly form H2SO4, but the reaction is so violent and inefficient that it can not be a practical source of sulfuric acid so, SO3 is reacted with H2SO4 to form oleum.Option D is elemental sulfur, which is used to make SO2 upon reacting with oxygen in the contact process.