Most large organisms that live on sediment-covered shorelines are called infauna. What does this ...

Most large organisms that live on sediment-covered shorelines are called infauna. What does this word refer to?
 
  a) it refers to the fact that these organisms burrow into the sediment
  b) it refers to the fact that these organisms are soft-bodied because they are not required to live in a harsh, rocky shore environment
  c) it refers to the fact that these organisms migrate from low tide regions to high tide regions depending on the time of day
  d) it refers to the fact that these organisms prefer the reeds and plants that grow along sediment-covered shores, a characteristic that usually does not exist on rocky shores