Scientists believe that the earth's early atmosphere lacked gas because photosynthetic organisms did not yet exist

The right answer is C. Oxygen

During photosynthesis, there is consumption of carbon dioxide and water to produce glucose and dioxygen. Glucose is the "fuel" of its growth. Cellular respiration will consume some of this glucose and oxygen to form oxygen and water.

So, if the atmosphere was lacking oxygen, it's simply because there was no photosynthesis producing oxygen.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.

d we did this already