【单选题】

It used to take RedwoodCity,California police hours to locate gunfire. Now, they have a new tool that helps them find the gunfire quickly. This new tool is called the ShotSpotter. The police began using the ShotSpotter in 1995. They wanted to cut down on gunfire in their city.

One ShotSpotter 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 ShotSpotter really works well. It locates the gunfire within 20 to 40 feet plus or minus. RedwoodCity police are very happy with the ShotSpotter. They think it might have helped catch theD、C、sniper.
ShotSpotters cost a lot of money. One of them sells for $200,000.Despite the cost, ShotSpotters 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