challenge: What is the relationship between the heat capacity rate of a hot fluid and the same fluid when cold?
  
  A. The hot fluid's heat capacity rate is always equal to the cold fluid's
  B. The hot fluid's heat capacity rate can be greater than, equal to, or less than the cold fluid's
  C. The hot fluid's heat capacity rate is always less than the cold fluid's
  D. The hot fluid's heat capacity rate is always greater than the cold fluid's
  Answer: 