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

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. The changes in this field, from grasses to shrubs to trees, represent
(a) ecological succession (b) climax production (c) biological evolution (d) ecological adaptation