In this task, you are given two facts, and a multiple-choice question. Based on the given facts, answer the question with index of the correct option (e.g, "A").
Q: Fact1: boiling means change from a liquid into a gas by adding heat energy, Fact2: Propane can be either a liquid or a gas., Question: what is added to propane to change it through the boiling process (A) vapors (B) energy (C) pH (D) heat (E) fats (F) SO2 (G) fusion (H) oxygen
A:
D