The small intestine can contract both circularly $($squeeze towards center$)$ and longitudinally $($shorten$).$ How many layers of smooth muscle are required to perform these functions and why?

One. One layer controls both movements.

Two. One layer to contract circularly and one to contract longitudinally.

Two. One layer to counteract and one to expand back to the original shape.

Three. One for each of the layers of the intestine: inner, middle, and outer.

Four. Two for contracting circularly $($one for contraction and one for expansion$)$ and two for longitudinally $($one for shortening and one for lengthening.$)$