A Lab Technician Is Preparing to Dispose of a Rat

A lab technician is preparing to dispose of a rat and a frog that were euthanized earlier in the day for a scientific study.Although the two animals died at the same time, the rat has rigor mortis while the frog does not yet show any signs of it.What is the best explanation for this time difference in the onset of rigor mortis?

A) The rat has higher concentrations of myoglobin in its muscles than the frog.
B) All the frog's muscles are fast-twitch muscles.
C) The rat's muscles have a greater density of myofilaments than the frog's muscles.
D) The frog has lower concentrations of glycogen in its muscles than the rat.