In Smooth Muscle, Cross-Bridging Occurs Due to

In smooth muscle, cross-bridging occurs due to

A) Ca²⁺ binding to troponin.
B) Ca²⁺ binding to calmodulin.
C) myosin kinase adding a phosphate to myosin heads.
D) both Ca²⁺ binding to troponin and calmodulin.
E) both Ca²⁺ binding to calmodulin and myosin kinase adding a phosphate to myosin heads.