In my economics class, I struggle with a long run and short run. How would I solve these two? Could someone explain?

Q: Indicate all of the following situations that would make a firm leave a perfectly competitive market in the long run. Select one or more: O a. PRAVC O b. P = ATC O c PATC Check Q: Indicate all of the following situations that would make a firm enter a perfectly competitive market. Select one or more: O a. P > ATC O b. P