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]: Which type of organism is the best at fixing nitrogen?  
 (A) bacteria (B) fungi (C) protists (D) animals
[A]: A


[Q]: After a star runs out of most of its hydrogen fuel, it becomes hotter and more luminous. What stage does this represent in the life cycle of a star?  
 (A) quasar (B) protostar (C) red giant (D) white dwarf
[A]: C


[Q]: Which part of the body carries out the function of copying DNA?  
 (A) brain (B) heart (C) bones (D) cells
[A]:
D