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.

[EX Q]: The change in fur color of the Arctic hare from brown in the summer to white in the winter is an example of  
 (A) mimicry. (B) heredity. (C) diversity. (D) camouflage.
[EX A]: D

[EX Q]: A student plans to grow algal colonies in solutions with different concentrations of liquid fertilizer. Which is a clear and testable hypothesis to guide this investigation?  
 (A) Algae like fertilizer. (B) Fertilizer is good for algae. (C) Fertilizer improves the genetic traits of algae. (D) Algal growth rate increases with fertilizer concentration.
[EX A]: D

[EX Q]: What is the primary function of white blood cells in humans?  
 (A) carry oxygen (B) fight disease (C) grow tissues (D) remove waste
[EX A]:
B