Enthalpy changes of reaction are provided for the following reactions.

$$\begin{array}{*{20}{l}} {{\text{2C(s)}} + {\text{2}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_4}{\text{(g)}}}&{\Delta {H^\Theta } =  + {\text{52 kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \\ {{\text{2C(s)}} + {\text{3}}{{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_6}{\text{(g)}}}&{\Delta {H^\Theta } =  - {\text{85 kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}} \end{array}$$

What is the enthalpy change, in ${\text{kJ}}\,{\text{mo}}{{\text{l}}^{ - 1}}$, for the reaction between ethene and hydrogen?

$${{\text{C}}_2}{{\text{H}}_4}{\text{(g)}} + {{\text{H}}_2}{\text{(g)}} \to {{\text{C}}_2}{{\text{H}}_6}{\text{(g)}}$$

A.     –137

B.     –33

C.     +33

D.     +137

A