You are given a science question (hard-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.

Q: Electric circuits provide energy for lightbulbs. Which of these prevents the flow of electrons?  
 (A) a circuit that is closed (B) a series circuit (C) a circuit that is open (D) a parallel circuit

A: C
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Q: Which of these most likely has the GREATEST mass?  
 (A) Chicken (B) Puppy (C) Lizard (D) Horse

A: D
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Q: At the beginning of a science class, the teacher opened a small sealed jar and placed it on a table. The contents of the jar spread throughout the room during class. Which of the following did the jar most likely contain?  
 (A) cloth fibers (B) oxygen gas (C) saltwater (D) snowflakes

A:
B
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