Where is the engineer who realized the ancient-dream from

A．the airliners of today would eventually be replaced by helicopters
B．helicopters would someday be able to transport large number of people from place to place as airliners are now doing
C．the imaginations fired by the Russian engineer's invention would become a reality in the future
D．their fantastic expectations about helicopters could be fulfilled by airliners of today

A．a combination of rotating devices in front and on top.
B．a rotating device topside.
C．one rotating fan in the center of the aircraft and others at each end.
D．a rotating fan underneath for liftin