Fact 1: Shivering occurs when many muscles contract a little bit all at once. Fact 2: Almost all birds use shivering as a short term adjustment to the cold. Given the two facts above, answer the question "What can contract a little bit all at once as a short term adjustment to the cold?" with the following options: - that metal - muscles - Weight - stomach - mammals - atoms - ligaments - sweating
A:
muscles