Given the task definition and input, reply with output. 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").

Fact1: condensation is when change from a gas into a liquid by reducing heat energy, Fact2: Clouds form by the condensation of water vapor., Question: Water vapor is formed when gas changes into a liquid by reducing what? (A) direct energy (B) electrical energy (C) kinetic energy (D) heat energy (E) solar flares (F) Temperature (G) energy (H) water power
D