What causes rigor mortis, the stiffness observed in the muscles of the deceased?

A) Actin is no longer responsive to myosin.
B) The sarcoplasmic reticulum no longer sequesters calcium, leading to continued contraction.
C) Troponin is unable to move tropomyosin off the active site on actin.
D) The sarcoplasmic reticulum no longer releases calcium, causing continued contraction.
E) The body ceases to produce ATP, essential for detaching myosin heads from actin.