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: The ocean minimum zone, or OMZ, is an area from 200 to 600 meters below the ocean's surface. The OMZ is dark and cold, with oxygen concentrations less than 10% of concentrations in surface water. Oxygen concentrations are so low because of bacterial activity that breaks down organic material falling from surface waters. Given the conditions in the OMZ, which of these dissolved gases would you expect to find in large concentrations in this zone?
(A) argon (B) nitrogen (C) carbon dioxide (D) carbon monoxide

SOLUTION: C

PROBLEM: Campers shouted toward a canyon wall. What characteristic of sound best explains why the campers heard an echo?
(A) Sound waves traveled slowly down the canyon. (B) Sound waves were absorbed by the canyon wall. (C) Sound waves reflected off the canyon wall. (D) Sound waves were refracted around the canyon.

SOLUTION: C

PROBLEM: Which best describes the characteristics of a river basin?
(A) the land drained by a river and its tributaries (B) the land formed when rivers create estuaries and marshes (C) the land at the mouth of a river where water flows into the ocean (D) the land formed as a result of a river flooding

SOLUTION:
A