Which part of the atom has to stay the same for it to be the same element?

a.) number of neutrons
b.) number of protons
c.) number of electrons
d.) number of protons, neutrons, and electrons

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You throw a small rock straight up from the edge of a highway bridge that crosses a river. the rock passes you on its way down, 7.00 s after it was thrown. what is the speed of the rock just before it reaches the water 28.0 m below the point where the rock left your hand? ignore air resistance.