A scientist testing the effects of a chemical on apple yield sprays an orchard with the chemical. A second orchard does not receive the chemical. In the fall, the number of apples harvested from each orchard is counted. Which of the following variables must be controlled in this experiment?
1. the amount of water that each orchard receives
2. the species of trees in the orchard
3. the number of apples that are produced by each tree
4. the size of each of the orchards

1 and 2

Explanation:

1) The exercise scientist is looking for the effects of a chemical between an apple crop to which it is administered and another to which it is not, 4 options are presented, of which it is essential to count as a variable the amount of water each Orchard and tree species in the orchard, since they can generate alterations in the results, the other two variables of the exercise such as number of apples and size of the orchards are not significant and their variations do not affect the scientist's objective.