Seven candidates appeared at the third of the mayoral forums sponsored by the Daily News and the Metro Industrial Areas Foundation citizens group. The topic was public safety. All were advised to come prepared to explain how he or she would hold the line on crime or drive it still lower — the central duty of any mayor. All were given the opportunity to present their three most important strategies. Not one of them was convincing or spoke with a semblance of coherence or authority. Since 1990, the city has enjoyed a steady downward trend in felonies. Under Mayor Bloomberg and Police Commissioner Ray Kelly, the NYPD has reduced crime to record levels, an achievement that strengthened the foundation of the city’s resurgence. Allow fear to return, and all will be lost, including lives.