TASK DEFINITION: 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.
PROBLEM: In which case does a physical change occur?  
 (A) Ice melts into water. (B) Bread burns in a toaster. (C) A cake bakes in an oven. (D) Two chemicals form a new substance.

SOLUTION: A

PROBLEM: What gas in the air do we need to breathe in order to live?  
 (A) nitrogen (B) oxygen (C) carbon dioxide (D) hydrogen (E) water vapor

SOLUTION: B

PROBLEM: A student was given the volume and description of a substance and was asked to determine its density. What information is missing?  
 (A) size (B) mass (C) boiling point (D) melting point

SOLUTION:
B