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.
--------
Question: Two forms of boron are naturally occurring, boron-10 and boron-11. What difference exists between a boron atom with a 10 atomic mass units (amu) and a boron atom with 11 atomic mass units (amu)?  
 (A) amount of neutrons (B) number of electrons (C) size of protons and neutrons (D) size of electrons and electron cloud

Answer: A


Question: Water in the solid phase has a crystal structure which keeps molecules farther apart than water at certain temperatures in the liquid phase. This relationship explains which property?  
 (A) the presence of more impurities in ice than in liquid water (B) the ability of ice to float on liquid water (C) the release of heat when liquid water turns into ice (D) the cooling of liquid water as it flows over ice

Answer: B


Question: What unit do scientists use to measure the distance between stars?  
 (A) light-year (B) angstrom unit (C) astronomical unit (D) apparent magnitude

Answer:
A