Rigor mortis is the stiffening of the body after death. After death, the body begins to get cold. From about two to six hours after death, the muscles start to stiffen, probably because the actin and myosin filaments of the muscle slide into each other. The rigor mortis starts at the eyelids, jaw and neck and then slowly spreads to all parts of the body within six hours.
Contrary to popular belief, rigor mortis will occur more quickly in high ambient temperatures than in cold conditions. This is because the underlying chemical reaction occurs faster at higher temperatures.
Oxygen was first produced via photosynthesis as ocean microbes evolved in the event that was called the Great Oxidation Event. Therefore the correct answer is C
