What does it mean by "researchers painstakingly tuned the design... " (Lines 3—4 Para. 5)

A．The workers on the underground platforms were replaced by machines.
B．It became the first completely automatic railway in the world.
C．A completely automatic line was added to its network.
D．Computers began controlling the trains on the line.

A．Waved their hands.
B．Shout and cried.
C．Waved their shirts.
D．Made a fire.