challenge: How much can a Differential Global Positioning System (DGPS) increase the accuracy of positional data?
  
  A. About tenfold, from 15 metres to 1.5 metres
  B. About ten thousandfold, from 15 metres to 0.0015 metres
  C. About a thousandfold, from 15 metres to 1–3 centimetres
  D. About a hundredfold, from 15 metres to 0.15 metres
  Answer: 