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option C. Addition of protons to the atomic nucleus.

Explanation:

Elements are fundamentally made up of atoms. Atoms in turn has three sub particles.

These particles are; neutrons, electrons and protons.

The sub particles responsible for the mass of an atoms is basically the protons and neutrons. Going through the options;

The correct option is option C. Addition of protons to the atomic nucleus.

a. a primary source combines several different accounts of an event. b. a primary source generally benefits from having been written after an event.

explanation:

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