What Is Responsible for the Phenomenon in Skeletal Muscle Known

What is responsible for the phenomenon in skeletal muscle known as muscle tone?

A) the passive elastic properties of muscles and joints and some degree of alpha motor neuron activity
B) spontaneous entry of calcium into the cytosol and the background phosphorylation of cross-bridges
C) depletion of ATP in some individual muscle fibers, and the inability for cross-bridges to detach
D) cross-bridges that continuously cycle, even in the absence of cytosolic Ca²⁺
E) a buildup of K⁺ in the T-tubules and resulting depolarization of the sarcolemma