The words "make" and "bus" are called ______ because they can occur unattached.

A．Human ancestors became predominantly right- handed when they began to use tools.
B．It is difficult to interpret the significance of anthropological evidence concerning tool use.
C．Human and their ancestors have been predominantly right - handed for over a million years.
D．Human ancestors were more skilled at using both hands than modern humans.

A．ancient artwork
B．asymmetrical skulls
C．studies of tool use
D．fossilized hand bones