Combine the roles of calcium and calmodulin to explain their impact on cellular signaling.

• A. Calmodulin directly phosphorylates target proteins without calcium.
• B. Calmodulin converts calcium into DAG.
• C. Calcium inhibits calmodulin, preventing activation of protein kinases.
• D. Calcium binds to calmodulin, forming a complex that activates various protein kinases.

Answer: (D)