At first the ______ of color pictures over a long distance seemed impossible, but, with painstaking efforts and at great expense, it became a reality.

A．canceled
B．dissipated
C．discarded
D．concealed

A．They were being kept prior to the seventeenth century.
B．They mention only epidemics of smallpox.
C．They provide quantitative and qualitative evidence about Native American populations.
D．They prove that certain diseases were unknown in the pre-Columbian New World.