challenge: Where is the dam system, comprising the Paulushofdamm and Obersee, located?
  
  A. In the Hunsrück mountains, northeast of the Moselle River
  B. In the Black Forest, west of the Danube River
  C. In the Rur Eifel, southwest of the Kermeter
  D. In the Taunus Mountains, south of the Lahn River
  Answer: 