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: The shape of a riverbed changes over time as a result of which gradual process?  
 (A) erosion (B) dissolving (C) plate shifting (D) compaction

A: A
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Q: Scientists theorize that the original atmosphere of Earth was stripped away by the solar wind shortly after the formation of the planet. The Jovian planets are far enough from the Sun that the solar wind has less of an impact. Studies of the Jovian planets suggest the early atmosphere of Earth was mainly composed of which two gases?  
 (A) oxygen and argon (B) hydrogen and helium (C) carbon dioxide and nitrogen (D) sulfur dioxide and water vapor

A: B
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Q: The boiling point of nitrogen is -196°C. What states of matter are involved when nitrogen boils?  
 (A) solid to gas (B) liquid to gas (C) liquid to solid (D) gas to liquid

A:
B
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