Multiple choice I genuinely have no idea-- but I don't think it's the first one or the last.
I think it may be the third one but I'm not too sure.

The correct option is;

By first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of reaction 2

Explanation:

The given reactions are;

1) C₂H₂ (g) +  2H₂ (g)  → C₂H₆ (g)          ΔH = -75 kcal

2) C₂H₄ (g) +  H₂ (g)  → C₂H₆ (g)          ΔH = -33 kcal

3) C₂H₂ (g) +  H₂ (g)  → C₂H₄ (g)          ΔH = ?

By reversing reaction (2) and subtracting the reactants from the products we then have followed by adding the enthalpy of reaction 1 to the value of the enthalpy of the (reversed) reaction 2, we have;

C₂H₂ (g) + 2H₂ (g)  → C₂H₆ (g)          ΔH = -75 kcal

C₂H₆ (g) → C₂H₄ (g) + H₂ (g)             ΔH = 33kcal

Adding, and cancelling out the italicized terms which are repeated in the product and reactants, we get

C₂H₂ (g) + H₂ (g) → C₂H₄ (g)         ΔH = -75 kcal + 33 kcal = -42 kcal

Therefore, the correct option is "by first reversing Reaction 2 and then adding the enthalpy of Reaction 1 and the enthalpy of reaction 2"

coral.

explanation:

coral. many marine organisms that produce calcium carbonate shells or skeletons are negatively impacted by increasing co2 levels and decreasing ph in seawater.

why is arctic sea ice important? arctic sea ice keeps the polar regions cool and moderate global climate. sea ice has a bright surface; 80 percent of the sunlight that strikes it is reflected back into space. as sea ice melts in the summer, it exposes the dark ocean surface.