Analyze the role of calcium ions in muscle contraction and relaxation.

• A. Calcium ions are stored in the cytoplasm during muscle relaxation.
• B. Calcium ions are irrelevant to muscle contraction and relaxation.
• C. Calcium ions bind to ATP to initiate muscle contraction.
• D. Calcium ions increase cytoplasmic concentration to induce contraction and decrease for relaxation.

Answer: (D)