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: A student is trying to identify a mineral that has a nonmetallic luster and is black. It can also be scratched with a fingernail. According to the mineral reference sheet, the unidentified mineral is most likely  
 (A) mica. (B) magnetite. (C) hornblende. (D) quartz.

A: A
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Q: Researchers recently discovered a gene in humans that previously was unknown to science. Which statement best explains why the gene probably was not discovered much sooner?  
 (A) The gene only recently evolved in humans. (B) The technology used to study genes is still being developed. (C) Scientists were not interested in genes until a few years ago. (D) Scientists were sure they had already discovered every possible gene.

A: B
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Q: Mercury, the planet nearest to the Sun, has extreme surface temperatures, ranging from 465°C in sunlight to -180°C in darkness. Why is there such a large range of temperatures on Mercury?  
 (A) The planet is too small to hold heat. (B) The planet is heated on only one side. (C) The planet reflects heat from its dark side. (D) The planet lacks an atmosphere to hold heat.

A:
D
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