challenge: How was Actinium(III) sulfide prepared?
  
  A. By heating actinium(III) oxalate at 1000°C for 1 hour in a mixture of water and sulfuric acid
  B. By heating actinium(III) oxalate at 1400°C for 6 minutes in a mixture of carbon disulfide and hydrogen sulfide
  C. By electrolyzing a solution of actinium(III) nitrate
  D. By reacting actinium(III) chloride with sodium sulfide at room temperature
  Answer: 