TASK DEFINITION: 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.
PROBLEM: Plants growing in a pond release a gas that fish need to survive. Which gas do the plants release that fish use?
(A) helium (B) hydrogen (C) nitrogen (D) oxygen

SOLUTION: D

PROBLEM: A leaf is covered in mud during a landslide. After one million years, this leaf will most likely be ___.
(A) a fossil (B) an animal species (C) a new plant species (D) a nonrenewable fuel

SOLUTION: A

PROBLEM: A researcher found shark fossils on top of a mountain. This evidence suggests which of the following about this region?
(A) It was once below a waterfall. (B) It was once part of a riverbed. (C) It was once covered by an ocean. (D) It was once near a freshwater lake.

SOLUTION:
C