A study is underway to determine the average height of all 29,000 adult pine trees in a certain national forest. The heights of 600 randomly selected adult pine trees are measured and analyzed. The sample in the study is: a. the average height of all the adult pine trees in this forest b. the average height of the 600 randomly selected adult pine trees c. all the adult pine trees in this forest d. the 600 adult pine trees selected at random from this forest