If the Spiral Density Wave Were the Only Thing Producing

If the spiral density wave were the only thing producing spiral arms, it would be expected that

A) all spiral arms would be gas and dust free.
B) all galaxies would have only two smooth spiral arms.
C) the Milky Way would show and irregular pattern of short arms .
D) the Milky Way wouldn't have any spiral arms.
E) the halo component of the Milky Way would show spiral arms as well.