The author is trying to claim that the rubbish pits provide little actual insight into how the people who lived there lived their lives. The author thinks this is because the pits have been eroded, and thus the actual rubbish had been degraded. B provides another way that the actual evidence has been destroyed. If rats had taken all the bones from the rubbish, then we would be systematically missing a key live of evidence. This strengthens the idea that the pits are actually not particularly strong evidence.