Multicellular organisms arose because a a part could die and be removed b they could eat unicellar organisms or c unicellar organisms began to die offwith the oxygen filled atmosphere

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.