Question

To pinpoint an earthquake's location, scientists need information from how many seismometers?

Answer 1

C. 3
   Due to the different speeds of P and S waves, a single seismometers can determine the distance to an earthquake. So, for a single station, the localization is any point on a circle around the station. With 2 stations, you'll have two circles that intersect at two points. The 3rd station is needed in order to determine which of the 2 points is the actual earthquake.