Cameron immediately rules out Stan Cantone due to the nails that were painted over. When he reads the part where Grace Copeland said her husband was doing jobs for the tenants, he re-reads the bit with Phyllis Waters again and realises there wasn't any sign of her actually hanging up the curtains in the room while she claimed to have been interupted and wouldn't have had any time to clear it up. He concludes that Phyllis Waters must have done it and hands in the quiz.