Living cells use as their main source of energy.

Carbohydrate (glucose)

Explanation:

Living cells require a constant supply of energy to generate and maintain their life processes. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells. Glucose, a sugar and simple carbohydrate, dominates energy production in most cells. Glucose is slowly broken down (catabolism) in a series of steps including- Glycolysis, Kreb's cycle, oxidative phosphorylation, collectively called CELLULAR RESPIRATION to produce ATP.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?

i believe the answer is snake.