根据以下资料，回答18~21题。 It used to take Redwood City, California police hours to locat

e gunfire．Now, they have a new tool that helps them find the gunfire quickly．This new tool is called the Shot Spotter, The police began using the Shot Spotter in 1995．They wanted to cut down on gunfire in their city. One Shot Spotter covers a square mile．Eight microphones are put on the roofs of buildings．These microphones pick up gunfire noise．It only takes seven seconds to report the gunfire to the police．Then a red dot on the police map shows where the action is. The Shot Spotter really works well．It locates the gunfire within 20 to 40 feet plus or minus．Redwood City police are very happy with the Shot Spotter．They think it might have helped catch the D.C．sniper. Shot Spotters cost a lot of money．One of them sells for $200,000．Despite the cost, Shot Spotters can be a big help in fighting crime. Eight microphones placed on rooftops __. A．talk to police B．warn the public C．listen for gunfire D．play rap music