Study the reactions. C2H2 (g)+2H2 (g)→C2H6 (g)ΔH=−75 kcal
C2H4 (g)+H2 (g)→C2H6 (g)ΔH=−33 kcal
C2H2 (g)+H2 (g)→C2H4 (g)ΔH= ?

How can the enthalpy of Reaction III be calculated correctly?

A) by first multiplying the enthalpy of Reaction 2 by two and then finding the sum of the enthalpy of Reaction 1 and the enthalpy of Reaction 2

B) by first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of Reaction 2
C) by adding the enthalpy of Reaction 1 and the enthalpy of Reaction 2
D) by subtracting the enthalpy of Reaction 1 from the enthalpy of Reaction 2