Which of the following statements is true?
a. modal jazz is based on a single scale for long periods of time.
b. modal jazz includes rapid chord changes.
c. modal jazz does not allow musicians to develop and explore melodic possibilities.
d. modal jazz never really caught on as a viable approach to jazz improvisation.

a. Modal jazz is based on a single scale for long periods of time.  

Explanation:

 Model Jazz is a type of jazz music that was developed in the later period of 1950s.

Modal jazz uses modal scales for slow harmonic rhythm improvising and pedal points to suppress functional harmonic relationships. It is basd on very few static harmony or chords progressions. It is based on a single scale for a longer period of time.

Modal jazz lay importance on improvising with fewer pitches to select from.

Hence the answer is --

a. Modal jazz is based on a single scale for long periods of time.  

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explanation:

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