Why did the portable radio play a key role in the popularization of rock ‘n roll?
选项：

A:. Because more people could hear the new songs as they were released.
B:. Because musicians could start their own radio stations and play the music they liked
C:. Because it was small, inexpensive, and portable, parents had less control over what their children listened to
D:. Because the radio was viewed as a symbol of freedom, so parents encouraged their children to listen to it