The cattle were removed from a field and shrubs and bushes were 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. What once was an open pasture on a long abandoned farm has changed over time, without the direct influence of human. In this wooded region that was once pasture, a mature forest of towering trees has taken over. However, change is still apparent, as a large patch of young shrubs and small trees are growing where several large trees were toppled by a storm several years ago. Collectively, this wooded regions represents 

(a) a climax ecosystem 

(b) primary succession 

(c) a sustained biome 

(d) intermediate succession

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