What does it mean that Aaron Copland's "Hoe-Down" is composed in an "open" fashion?

a) He composed it in a public area where all could see him working.
b) He left some parts unwritten and "open" for improvisation.
c) He leaves sonic space between instrumental parts, much like the open space of the American West.
d) He chose to leave the piece open for later compositional additions and improvements.

c