贯彻“三个代表”重要思想，关键是[ ]

A．坚持学习

B．吸收世界文明成果

C．不断进行理论创新

D．坚持与时俱进

．No one knows exactly when our ancestors started talking，but new evidence suggests  _______might have happened a long time ago．

A．which

B．what

C．it

D．they