X gets on an elevator at the 11th floor of a building and rides up at th erate of 57 floors per minute. At the same time, Y gets on an elevator at the 51st floor of the same building and rides down at the rate of 63 floors per minute. If they continue travelling at these rates, then at which floor will their paths cross?

• A. 29
• B. 30
• C. 31
• D. 32