He came back late,______ which time all the guests had already left.

A．in toy-making there is a continuity in the use of materials
B．even the simplest toys can reflect the progress of technology
C．it often takes a long time to introduce new technology into toy-making
D．even a simple toy can mirror the artistic tastes of the time

A．follow a direct line of ascent
B．also appeal greatly to adults
C．are not characterized by technological progress
D．reflect the pace of social progress