The answer is D) rhythm.
During the Ars Antiqua, music theory made great strides. One of the most important innovations from this time was the devlopment of a notation, or written representation in sheet music, of rhythm. Rhythm is naturally a very important element of music, and being able to represent the rhythm of a song in written form made it much easier to play it accurately without having heard it.
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I would say a c d and e i hope this helps
