How could the early 60s be characterized in the evolutionary history of rock
a) A period marked by the decline of rock's popularity due to the emergence of other genres.
b) An era of experimentation and diversification, with the coexistence of various subgenres and styles within rock music.
c) A time when rock remained stagnant, resisting change and innovation.
d) A phase dominated by a single dominant subgenre, overshadowing other musical styles within rock.