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

A．She Was too busy to remember if she had returned them.
B．She didn’t return them in fact.
C．She gave them to the landlord to return.
D．She did return them but somehow the library made a mistake.

A．A platform worker.
B．A computer.
C．The command spot.
D．A machine.