The independent variable is the plant growth or how much the plants grow.
Plant Root and Shoot System. Plants have two Organ Systems: the shoot system and the root system. Stems have many jobs, including supporting the plant; acting like the plant's plumbing system, conducting water and nutrients from the roots and food in the form of sugar (glucose) made in the leaves to other plant parts.
The best and most correct answer among the choices provided by your question is the first choice.
The best analogy would be: the body storing energy is like a car holding gas: the car's gas tank keeps the gas until it is needed.
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