challenge: What is the composition of the hypothetical compound Zirconocene?
  
  A. Two cyclohexyl rings bound on a central titanium atom
  B. Two cyclopropyl rings bound on a central titanium atom
  C. One cyclopentadienyl ring bound on a central zirconium atom
  D. Two cyclopentadienyl rings bound on a central zirconium atom
  Answer: 