What Happens in the Bar of a Barred Spiral

What happens in the bar of a barred spiral?

A) The bar rotates like a rigid body with no relative motion of the stars and gas inside.
B) The bar is part of the spiral arm structure, and it is actually a shock wave moving through the relatively rapid circular motion of the individual stars.
C) Stars tend to enter one end of the bar, pass through the it, and exit at the other end.
D) Stars and gas tend to move back and forth from one end of the bar to the other.