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.

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before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.