Which of the following is a compound sentence?
A. Butterflies and hummingbirds feed on the nectar of flowering plants.
B. Because fraud is so common in some corporations, regulatory agencies are overworked.
C. Because Viceroy Butterflies migrate over very great distances, tracking them is a challenge to entomologists.
D. Corporate fraud is becoming more common in the United States, and the civil courts are being stretched to their limits.