Instructions: You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Input: When two nuclei are combined into one nucleus, there is a slight change in mass and the release of a large amount of energy. What is this process called?
(A) conversion (B) reaction (C) fission (D) fusion
Output:
D