26) E

Since temperature is proportional to pressure, pressure will initially increase to allow both gases to react.

Now let us write out the reaction equation

C₂H₄ + H₂  ->  C₂H₆

Using the equilibrium rules, we can see that moles of products is ½ of moles of reactants. This means that as the reaction progresses, pressure will decrease.

Hence, E is the answer.