Breland's Trained Raccoons Would Deposit Single Tokens into a Slot

Multiple Choice

Breland's trained raccoons would deposit single tokens into a slot but would not deposit two tokens,which they rubbed together.What explanation did the Brelands use for this behaviour

A)  instinctive drift toward food-washing behaviour 
B)  insufficient reinforcement of token placing 
C)  lack of interest in the single tokens 
D)  poor conditioning of token placing