Scientists once believed that protons, electrons, and neutrons were the smallest particles of matter. Now they believe that these particles are made of even smaller particles.Which statement gives one of the reasons the original belief was a theory?

A scientific theory is a widely accepted explanation supported by evidence, not simply a guess. The belief that protons, electrons, and neutrons were the smallest particles was an accepted explanation of matter’s structure at the time, so “a widely accepted explanation of the structure of matter” is the correct choice. Later discoveries changed that explanation as scientists found smaller particles.

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