With regard to sound, what is the crucial difference between sound and silent films?

$$ a$)$ A sound film can emphasize silence, but a silent film has no option.

b$)$ A sound film only uses diegetic sound, but a silent film can use nondiegetic sound.

$$ c$)$ A sound film uses automatic dialogue replacement $($ADR$),$ but a silent film uses rerecording.

$$ d$)$ A sound film's fidelity is never faithful to its source, and silent films are always faithful to their sources.