The activity of animals is usually coordinated with periodically recurring events in the environment. A. Most animals survive and reproduce successfully without coordinating their activities to external environmental rhythms. B. The circadian period of an animal's internal clock is genetically determined and basically unchangeable. C. Environmental cues such as a change in temperature are enough to reset an animal's clock. D. Animals have internal clocks that influence their activities even when environmental cues are absent. E. Animals are less affected by large differences between their internal rhythms and the local solar day than are humans. F. Because an animal's internal clock does not operate on a 24-hour cycle, environmental stimuli are needed to keep the biological day aligned with the solar day.