challenge: What was the average speed of the first services on the Glasgow, Paisley, Kilmarnock and Ayr Railway line in 1839?
  
  A. 15 miles per hour (24 km/h)
  B. 21 miles per hour (34 km/h)
  C. 30 miles per hour (48 km/h)
  D. 40 miles per hour (64 km/h)
  Answer: 