The answer is observations
Answer:
D
Explanation:
A robin building a nest is not learned or taught like the others. It is just instinctive for robins to build nests.
Hormones are chemical substances that help to regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the bloodstream. Hormones can be used to control human fertility.
The direct source of energy to move the sailboat is probably wind, since wind is what pushes the sail of the sailboat, which is what propels the sailboat forward.
Hope this helps!
