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]: Which object is best used to demonstrate the bending of light as it passes through a substance?  
 (A) convex lens (B) flat mirror (C) opaque rod (D) concave mirror
[EX A]: A

[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]: The piping plover is a bird that builds its nest on sand dunes along the Atlantic Ocean. Which of these actions most likely damages the nests of the piping plover?  
 (A) people fishing from a boat (B) children playing in the ocean waves (C) children digging holes along the sand dunes (D) people walking on a sidewalk along the sand dunes
[EX A]:
C