S7. Thanks to ______ , rock music is widely welcomed by people around the world. Most important object of study is.

A．Providing a warning light when the sound is too loud.
B．Producing more personal stereo systems.
C．Restricting the use of personal stereos.
D．Setting up a standardized hearing test.

A．They live longer.
B．They eat more salt.
C．They have changed their sleeping habits.
D．They eat less fruit.