What is the significance of "idempotency testing" for APIs, particularly in distributed systems with retries?

A It verifies that repeating the same request (e.g. due to a network retry) does not produce unintended side effects like duplicate records or double charges B It checks that an API always returns a different result each time it is called C It only applies to read-only GET requests and never to writes D Idempotency testing is unrelated to retries or distributed systems