What Is the Motion of Stars in the Bar Across

What is the motion of stars in the bar across the inner part of the Milky Way Galaxy?

A) The stars move along one side of the bar and then come back along the other side.
B) The stars oscillate back and forth along the bar, retracing the same path again and again.
C) The stars are part of a density wave which moves around the galactic center. The bar simply represents the place where stars and dust are most concentrated.
D) The stars basically remain stationary within the bar. The bar itself moves slowly around the galactic center, requiring more than 200 million years to orbit once.