Which of the following equations represents a substitution reaction?

• A C ₂H_10(g) + CI _2(g) → C₄H₉ CI_(g) + HCI_(g)
• B C ₂H_4(g) + HCI _(g) → C ₂H₅ CI_(l)
• C C₂H_2(g) + 2H _2(g) → C ₂H_6(g)
• D C ₃H_4(g) + 40_2(g) → 3CO_2(g) + 2H ₂O_(g)