It means an organism respire without it
so that organism know as anaerobic
for example : bacteria eats human and animal dead bodies under the earth there O² is deficient so bacteria respire with oxygen O²
The heart rate would decrease because there is less oxygen being supplied to the cells for cellular respiration, and also your muscles won't get the blood it needs. Blood also contains oxygen and glucose which cells need to convert food to energy and to survive, so if the cells don't get blood they will die which will eventually kill the organism
The answer is D.rainforest. it is a very complex ecosystem with variety of plants and animals.
