challenge: What is the primary mechanism used in early Davis sewing machines, known as their 'Vertical Feed' machines?
  
  A. Conventional four motion feed with feed dogs
  B. A rotary motion that lifts the cloth to meet the needle
  C. Two presser feet, with one moving the cloth while the needle is through it
  D. A single presser foot that holds the cloth stationary at all times
  Answer: 