Allows manipulation of the environment, locomotion, and facial expression. Maintains posture, and produces heat.

• A. Muscular system
• B. Nervous system
• C. Skeletal system
• D. Endocrine system

Answer: (A)

Movement and heat production come from the muscular system. Skeletal muscles attached to bones contract to pull on the skeleton, letting you manipulate the environment, walk or run, and make facial expressions with the muscles of the face. Maintaining posture relies on continuous, low-level muscle tone to keep joints aligned against gravity. Heat is generated as muscles use energy during contraction and metabolism, helping to keep body temperature stable. The nervous system coordinates these actions by signaling muscles to contract, but the actual movement and heat come from the muscles themselves. The skeletal system provides the framework but doesn’t contract or generate heat, and the endocrine system influences metabolism and heat production more indirectly.