What was once an open pasture on a long-abandoned farm has changed over time, without the direct ...

What was once an open pasture on a long-abandoned farm has changed over time, without the direct influence of humans. After the cattle had been removed, shrubs and bushes could be seen dotting the grass-covered hillsides.
 
  A few years later, small pine trees and then larger deciduous trees appeared. Now, there are so many trees and shaded regions that it is difficult to tell that this was once a field. The ecological changes in this abandoned pasture represent ________.
  A) primary succession B) secondary succession
  C) disturbances D) resilience events