Which of the following comparisons should be made to determine whether the scientists' hypothesis is correct?

The scientists hypothesize that "deer affect gene expression in holly tree leaves." To test this, an experiment needs to compare holly tree leaves exposed to deer with holly tree leaves not exposed to deer, while keeping other variables constant.

• A) Compares roots to leaves. This is not relevant to the hypothesis about leaves and deer.
• B) Compares two trees in the same area with deer. This doesn't provide a control group without deer.
• C) Compares gene expression in holly tree leaves in an area with deer to gene expression in holly tree leaves in an area with no deer. This directly isolates the variable of deer presence/absence on the target tissue (holly tree leaves), making it a suitable comparison to test the hypothesis.
• D) Compares holly tree leaves to leaves of another tree species. This introduces an additional variable (different tree species) that would confound the results, making it difficult to attribute any differences solely to the deer.

The correct option is C).