A scientist testing the effects of chemical fertilizer on apple trees sprays part of an orchard (50 trees) with the fertilizer. Another part of the orchard (also 50 trees) does not receive the fertilizer. In the fall the number of apples harvested from each part of the orchard is counted. The part of the orchard with the chemical fertilizer averaged 62 apples per tree. The part of the orchard without the fertilizer averaged 45 apples per tree.

In order for this test to be valid (fair) both orchards must:

A. receive different amounts of fertilizer

B. have different species of apple trees

C. receive different amounts of water

D. have the same species of apple trees