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.

My Conclusion - Once food is cut on Wooden board. Bacteria just gets burrowed in the wood. So the Surface looks "uninfected'. So, We dont need to wash. Wiping it is good enough. That means the author is assuming that the bacteria that has pushed into wood. wont come out. If he thinks it emerges out of the wood. He'd want us to wash the wooden board as well. So answer is D