Choose the true statement.

• A. Smooth muscle depends on the calcium-calmodulin system to regulate contraction, while skeletal muscle relies on the calcium-troponin system to regulate contraction.
• B. Unlike skeletal muscle, smooth muscle does not depend on ATP to fuel contractions.
• C. Unlike skeletal muscle, contraction in smooth muscle does not involve a sliding filament mechanism.
• D. The final signal for contraction in smooth muscle is a rise in extracellular calcium, while the final signal for contraction in skeletal muscle is a rise in intracellular calcium.