Achievement of desired relative positions of the articulators can be achieved with a system that is metaphorically described by a pulley with weights on either side, corresponding, say, to the upper and lower lip. What this metaphor is referring to is a neural network that relies on

A) Excitation of the upper lip by an upper lip controller
B) Excitation of the lower lip by a lower lip controller
C) A bilabial closure unit that activates the upper lip controller and the lower lip controller, and also allows the upper and lower lip controllers to inhibit one another
D) All of the above