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: After a star runs out of most of its hydrogen fuel, it becomes hotter and more luminous. What stage does this represent in the life cycle of a star?  
 (A) quasar (B) protostar (C) red giant (D) white dwarf

SOLUTION: C

PROBLEM: Which difference between water and ice results in ice floating on cold water?  
 (A) heat (B) shape (C) density (D) volume

SOLUTION: C

PROBLEM: A galaxy is best described as a cluster of      
 (A) hundreds of stars. (B) thousands of stars. (C) millions of stars. (D) billions of stars.

SOLUTION:
D