What does mensuration indicate in early music notation?

a: rhythm
b: pitch
c: key signature
d: time signature

i think it's either a or b

The answer is A. Rhythm. 

Mensural notation is the musical notation system used for European vocal polyphonic music from the later part of the 13th century until about 1600. The term "mensural" refers to the ability of this system to describe precisely measured rhythmic durations in terms of numerical proportions between note values.

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