There are many farms that only grow pine trees. In the spring, seedlings are planted in rows that are ten feet apart. After four or five years, one-third of the trees are removed to give the others more room. Two or three years after that, another one-third are removed. Then, every other year the lower branches are cut away so that the trees' growth goes up and not out. Depending on the climate of the region, the trees may be harvested when they are between 20 and 35 years of age.
What is the main structure used in the passage?
1. chronology
2. comparison
з. cause and effect
4. problem and solution