Given the task definition and input, reply with output. 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.

When too much salt is detected in the bloodstream, the brain secretes a hormone that helps the body retain water by reducing the amount of urine produced. This is an example of
(A) a feedback loop. (B) an acquired trait. (C) a balanced ecosystem. (D) a symbiotic relationship.
A