Which single factor is tended to have been considered by the author as the most important in the development of man

A．Training given to music therapists.
B．How music prevents disease.
C．Studies on the benefits of music.
D．How musicians create music.

A．Workers on the platforms and trains were replaced by machines.
B．A completely automatic line was added to its network.
C．It became the first completely automatic railway in the world.
D．Its trains became manual-controlled.