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.

Q: Which of these is a structure that is present in all organisms?  
 (A) a tissue (B) a cell (C) a stomach (D) a brain

A: B
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Q: The burning of fossil fuels affects the atmosphere by  
 (A) adding more heat and carbon dioxide. (B) reducing the amount of necessary heat. (C) taking water vapor out of the atmosphere. (D) putting more oxygen in the atmosphere.

A: A
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Q: A glacier sometimes leaves behind large chunks of ice. What is left in the ground when these chunks of ice melt?  
 (A) a valley (B) a kettle (C) a crater (D) a sinkhole

A:
B
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