A study is under way to determine the average height of all 63,000 adult walnut trees in a certain national forest. The heights of 950 randomly selected adult walnut trees are measured and analyzed. The sample in this study is ________.
1) The average height of the 950 randomly selected adult walnut trees
2) The average height of all the adult walnut trees in this forest
3) All the adult walnut trees in this forest
4) The 950 adult walnut trees selected at random from this forest