Which of the Following Chemical Equations Does Not Correspond to a Standard

Which of the following chemical equations does not correspond to a standard molar enthalpy of formation?

A) Mg(s) + C(s) + 3/2 O₂(g) $\to$ MgCO₃(s)
B) C(s) + 1/2 O₂(g) $\to$ CO(g)
C) N₂(g) + O₂(g) $\to$ 2 NO(g)
D) N₂(g) + 2 O₂(g) $\to$ N₂O₄(g)
E) H₂(g) + 1/2 O₂(g) $\to$ H₂O( )