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]: A scientist combines oxygen and hydrogen to form water. This combination illustrates that water is  
 (A) an atom. (B) an element. (C) a mixture. (D) a compound.
[EX A]: D

[EX Q]: Cell membranes allow some molecules to move freely across the membrane, while other molecules are restricted. Which term best describes this capability of a cell membrane?  
 (A) semipermeable (B) impervious (C) resilient (D) unyielding
[EX A]: A

[EX Q]: Binary stars orbit around the center of mass of the two stars. What causes the stars to remain together?  
 (A) gravity between the stars (B) energy released from the stars (C) nuclear reactions in the stars (D) magnetic attraction between the stars
[EX A]:
A