There is a series of three chemical reactions (1→2→3) that form the overall chemical reaction. How is the Δh₃ calculated if the others are known?

a) Δh₃ = (Δh₁)(Δh₂)(Δh₄)
b) Δh₃ = Δh₄ - (Δh₁ Δh₂)
c) Δh₃ = Δh₁ Δh₂ - Δh₄
d) Δh₃ = Δh₁ Δh₂ Δh₄