The Dirty $($Modified$)$ bit used with Page replacement algorithms:

a$)$ is set to show which frames are filled with dirty $($garbage$)$ and free to be used for new pages.

b$)$ shows which frames hold a page that is modified and would be a very good candidate to be replaced

so that the modifications can be saved to the hard drive.

c$)$ shows which frames hold a page that is modified and would not be very good candidate to be

replaced because it may be more expensive to do so than other pages.

d$)$ shows a page is modified if set and therefore should never be replaced.