Phosphoric acid, which is commonly used as rust inhibitor, food additive and etching agent for dental and orthopedic use, can be synthesized using a two-step thermal process. in the first step, phosphorus and oxygen react to form diphosphorus pentoxide: (l) (g) (g)

The first step in the synthesis of phosphoric acid is the reaction of phosphorus and oxygen to form diphosphorus pentoxide.

The second step in the synthesis of phosphoric acid is the reaction of diphosphorus with water to form phosphoric acid.

The net chemical equation representing the formation of phosphoric acid from phosphorus, water and oxygen will be,

Equation in step 2 must be multiplied by 2 in order to cancel diphosphorus pentoxide.

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it is the most commonly accepted one

i think it's letter a.