Which of These Hypotheses Cannot Be Tested by an Experiment

Which of these hypotheses cannot be tested by an experiment?

A) Biological interactions between barnacle species are responsible for their distribution patterns.
B) Night-feeding fishes rely on their sense of smell to locate prey.
C) The growth of salt marsh grass is limited by nitrogen availability.
D) Certain humpback whale behaviors are important for attracting a mate.
E) Goldfish prefer to eat brightly-colored foods.