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.

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

[EX Q]: A dragonfly develops through incomplete metamorphosis. Which stages does the dragonfly's life cycle include?  
 (A) egg and larva (B) egg and nymph (C) larva and pupa (D) larva and nymph
[EX A]: B

[EX Q]: The chemical formula for water is H_{2}O. What does the formula show about each element in the compound?  
 (A) the number of valence electrons (B) the charge of the compound (C) the mass of protons in each atom (D) the number of atoms of each element
[EX A]:
D