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.

When oxygen combines with hydrogen, which substance is formed?  
 (A) water (B) vinegar (C) hydrochloric acid (D) hydrogen dioxide
A

What kind of energy conversion occurs after a candle is lit?  
 (A) friction to electrical energy (B) light energy to heat energy (C) mass to kinetic energy (D) chemical energy to light energy
D

Which type of matter can be separated by physical means?  
 (A) atom (B) element (C) compound (D) mixture

D