1. Testing only one variable
A controlled variable means its the one that stays the same. In a controlled experiment, it means there is only one manipulated, (changed) variable.
The answer to the question is possibly A
He found a nuclear structure (cells) in plants by looking through a microscope
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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