Why did composers of early polyphony use chant as the basis for their works?

Chant was believed to have originated from pope gregory the great who received it directly from God.

Explanation:

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including the octave, the answer would 8 notes in a major scale with one note being played twice. ex) c major scale includes: c, d, e, f, g, a, b, c

notice that c is played twice

however, if you were to ask me how many different notes are in a major scale- the answer would be 7.