Moss invades and establishes itself on bare rock, accumulating the beginnings of soil. After several years, enough soil has become established that grasses begin to grow where there was once bare rock. Without the moss building up soil, the grasses would have had no chance. The mosses changed the environment enough to permit grasses to grow in a process called:
(a) sublimation.
(b) facilitation.
(c) regeneration.
(d) improvisation.