challenge: What was the primary cause of the severe torsional drive train vibrations in the S-class submarines built by Electric Boat?
  
  A. Inadequate cooling systems leading to overheating
  B. Crankshafts of the NELSECO 8-EB-15 engines were of an insufficient diameter
  C. Insufficient lubrication in the engine's cylinder/piston system
  D. Misaligned propeller shafts causing uneven power distribution
  Answer: 