After an animal dies, its muscles start to stiffen before the decomposition of tissues relaxes the muscles again. Which of the following would you expect to explain this muscle stiffening (i.e. rigor mortis)?

A) The myosin II heads in muscle fibers remain attached to actin filaments due to the absence of Ca²⁺ in these cells.
B) The ATPase activity of muscle myosin II is inhibited by the elevated Ca²⁺.
C) The myosin II heads in muscle cells remain attached to actin filaments due to the elevated Ca²⁺ in the muscle fibers.
D) Titin molecules unfold, preventing muscle relaxation.
E) The myosin II heads in muscle cells remain attached to actin filaments due to covalent cross-linking.