5. For which one of the following reactions is the value of ∆Horxn equal to ∆Hof for the product?
(A)2Ca(s) + O2(g)→ 2CaO(s)
(B)C2H2(g) + H2(g)→ C2H4(g)
(C) 2C(graphite) + O2(g)→ 2CO(g)
(D)3Mg(s) + N2(g)→ Mg3N2(s)
(E) None of these