Bacteria from food can survive for several days on the surface of plastic cutting boards, but bacteria can penetrate wooden cutting boards almost immediately, leaving the surface free of contamination. Ttherefore, 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? ['Bacteria that penetrate into wooden cutting boards do not reemerge on the surface after the cutting boards have been used.', 'Washing plastic cutting boards does not remove all bacteria from the surface.', 'Prevention of bacteria contamination is the only respect in which wooden cutting boards are superior to plastic cutting boards.', 'Washing wooden cutting boards kills bacteria below the surface of the cutting boards.']