why DoEs thE proFEssor mEntion BonEs AnD shElls
A．to givE An ExAmplE oF nAturAl shApEs.
B．to DEsCriBE EArly sCulpturE.
C．to illustrAtE thEir usE As tools.
D．to DEmonstrAtE thEir rolE As DECorAtivE oBjECts.