Read the passage and choose the correct answer: The history of rock music has been volatile and unpredictable as the genre has constantly redefined and reinvented itself since its emergence in the late 1940s. Not surprisingly, then, it can be difficult to apply a straightforward definition to such a restless musical format. But while people might quibble over specifics, rock music can generally be described as hard-edged music performed with electric guitars, bass, and drums and usually accompanied by lyrics sung by a vocalist. That sounds simple enough, but a closer look at the evolution of rock suggests how different styles and influences have shaped its development over the years. Rock’s origins can be traced to the late 1940s, when the popular styles of the day, country music and blues, morphed into a new sound aided by electric guitars and a steady drumbeat. Pioneering rock artists of the 1950s such as Chuck Berry leaned heavily on classic blues structures while demonstrating a flair as natural-born entertainers. In contrast to the safe pop music of the era, rock’s aggressive attack suggested a sexual freedom that was shocking during that conservative age. By the early '60s, Berry’s followers, most notably the Rolling Stones, expanded rock’s scope by transitioning from singles artists into musicians capable of producing cohesive albums of songs. Embracing sex and youthful rebellion in their music, the Stones courted controversy but also elevated rock to new cultural heights. Sexual freedom is unpopular during that conservative age.