Which of the following statements about vaporization and steam is MOST accurate?

Which of the following statements about vaporization and steam is MOST accurate?
 
  A. Steam produced in the upper layer of hot smoke and fire gases may cause the upper layer to expand downward.
  B. Steam produced on contact with hot surfaces is more likely to result in complete vaporization of the fire stream.
  C. Steam produced in the upper layer of hot smoke and fire gases will cause the upper layer to contract toward the ceiling.
  D. Steam produced on contact with hot surfaces has little effect on the total volume of the upper layer of hot smoke and fire gases.