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: The Sun is classified as a star. Which characteristic identifies the Sun as a star?  
 (A) The Sun spins on its axis. (B) The Sun is located outside the Milky Way galaxy. (C) The Sun is part of the Big Dipper. (D) The Sun produces light and energy in a nuclear reaction.

A: D
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Q: A tropical rain forest contains many tall trees. Smaller plants with large leaves grow at the base of the tall trees. The large leaves are most likely an adaptation of the plant due to which condition?  
 (A) lack of sunlight (B) lack of oxygen (C) lack of water (D) lack of food

A: A
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Q: Why is it difficult to create a terrarium ecosystem that will last a long time without human help?  
 (A) Terrarium ecosystems cannot get enough sunlight. (B) Terrarium ecosystems cannot get enough oxygen. (C) Ecosystems with few species are weak and unstable. (D) All ecosystems require thoughtful human management.

A:
C
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