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]: In 2014, a break in a storm water pipe released thousands of gallons of coal ash from a retired electrical plant into a river. The spill occurred upstream from the water treatment plants that serve communities along the river. Coal ash contains several hazardous metals including arsenic. Which stage of the water treatment process is most likely to remove hazardous metals from the water?  
 (A) aeration (B) coagulation (C) disinfection (D) reverse osmosis
[A]: B


[Q]: A combination of processes occurs when stars are forming. Which is one process most likely associated with the formation of new stars?   
 (A) Hydrogen in the cores of the stars is exhausted. (B) Material accumulates from stars that have died. (C) Elements in the stars such as iron undergo fusion. (D) Cores of stars become twice as massive as the Sun.
[A]: B


[Q]: Water freezing is an example of a  
 (A) liquid changing to a solid (B) solid changing to a liquid (C) gas changing to a solid (D) gas changing to a liquid
[A]:
A