Physics
Conservation of energy refers to the fact that

If you conserve energy you will not be as tired
Energy cannot be created or destroyed but can be converted from one form to another
The entropy of the universe is always increasing
No chemical reaction is 100 percent efficient

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Physics
Isotopes of an element contain

the same number of protons but different number of neutrons
equal number of nucleons
equal number of protons and electrons
the same number of neutrons but different number of protons

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