To answer that question I would say A. source trait
Answer:
How the Muscular and the excretory system work together. The Muscular system helps the Excretory system by moving waste through the body and protect its organs. The Excretory system helps the Muscular system by getting rid of carbon dioxide from the muscles.
Explanation:
B.) A dog walks to a stream and drinks when it gets thirsty
would have to be your answer..