The truth of the matter is that who we are is the result of both heredity and the environment. It is not "nature versus nurture," but a combination of nature and nurture. I also want to point out that "behavior" does not seem to be a very precise term to use in this context, as behavior is simply what we do, which is not necessarily consistent over a lifetime or even over a day. Behavior is situational to a very large degree and seldom consistent enough to try to analyze it in this way. To the degree that this is the exact issue you are expected to address, behavior is environmental because behavior cannot be inherited at all. Our personalities, on the other hand, seem to be fairly consistent over our lives, and it is these that are a combination of nature and nurture.
Some traits, like shyness, are tendencies some people are born with—a genetic predisposition. If that predisposition is reinforced by one's environment, consistently, one is likely to become a shy adult. If that predisposition is not reinforced and a child is provided with opportunities to be more outgoing, he or she may become more outgoing with time. An inhibited child born into a rowdy family may retreat into even greater inhibition. A child who is gently exposed to the opportunity to be gradually more outgoing and uninhibited with very small birthday parties or family get-togethers is unlikely to be an extremely outgoing adult, but is not going to have to hide in the bathroom at a large event.
We are all born with various predispositions that can be enhanced or repressed by our environments. Our behaviors, on the other hand, are situational, as behavior is what we do, and there is nothing hereditary about taking a walk, attending a class, or washing the dishes.