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]: The shape of a riverbed changes over time as a result of which gradual process?  
 (A) erosion (B) dissolving (C) plate shifting (D) compaction
[A]: A


[Q]: Which statement best describes how birth and death rates compare in a population at carrying capacity?  
 (A) The birth rate is greater than the death rate. (B) The birth rate is less than the death rate. (C) The birth rate is equal to the death rate. (D) The birth rate and the death rate are zero.
[A]: C


[Q]: Students run laps around the track in physical education class. After a few laps, the students' breathing gets heavier, and their pace begins to slow. The students are most likely slowing down because  
 (A) they are exhaling carbon dioxide. (B) their lung cells are multiplying. (C) their respiratory system stops functioning. (D) they are getting less oxygen to their cells.
[A]:
D