What once was 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 it is difficult to tell that this was once a field. these ecological changes in the abandoned pasture represent

a secondary succession.
b aquatic to terrestrial succession.
c intermediate succession.
d primary succession.