Antagonistic Muscle Action in Arm Extension

Steps:

• Straightening the arm is extension, involving biceps and triceps as antagonists.
• Biceps (flexor) bends the arm; triceps (extensor) straightens it.
• During extension, biceps relaxes while triceps contracts.
• Option D matches: first muscle (biceps) relaxes, second (triceps) contracts.

Why D is correct:

• Antagonistic muscles work in opposition; the extensor contracts and flexor relaxes to enable extension, per basic muscle physiology.

Why the others are wrong:

• A: Incomplete; mentions only one muscle without specifying the pair's actions.
• B: Describes flexion, where biceps contracts and triceps relaxes.
• C: Incorrect terminology; muscles contract or relax, they do not expand.

Final answer: D