sea levels were much lower than they are now during the ice age. This is because water molecules bind together very tightly, creating a compact piece of ice. (The shape is dependent to the container of the water before frozen.)
The circulatory system and respiratory system are protected by your skin, muscles, and ribs.
Cell wall because their has to be something protecting it.
I think the correct answer from the choices listed above is the last option. It is the hormone oxytocin that is controlled by a positive-feedback mechanism. Oxytocin is normally produced by the paraventricular nucleus of the hypothalamus and released by the posterior pituitary.<span> Hope this answers the question.</span>