Final answer:
The H and O in living organisms come from oxygen obtained from the air and water, and hydrogen obtained from water. Carbon and nitrogen are obtained from the food we eat.
Step-by-step explanation:
The majority of the hydrogen (H) and oxygen (O) in living organisms comes from different sources. Oxygen is obtained from the air we breathe and the water we drink, while hydrogen is obtained from water. Carbon and nitrogen, on the other hand, are obtained from the food we eat. Although carbon is present in the atmosphere as carbon dioxide, and nitrogen makes up about 80% of the atmosphere, we obtain these elements from our food, not the air.