GMAT Critical Reasoning (CR) | Re: Bacteria from food can survive for several days on the surface of plas

Bacteria from food can survive for several days on the surface of plastic cutting boards, but bacteria can penetrate wooden cutting boards almost immediate in leaving the surface free of contamination. Therefore, wooden cutting boards, unlike plastic cutting boards, need not be washed in order to prevent their contaminating food that is cut on them; wiping them off to remove food debris is sufficient.
Which one of the following is an assumption on which the argument depends?
(r\) Washing plastic