Common Courtesy

How do you feel about "common courtesy"? Do you think it is important to learn? Is it an outdated practice? Do you even know what it is? How would you have reacted in this situation:
You go to the movies with a large group of friends, and take up almost a whole row of seats in the theater. Then, as more friends show up, you all scoot over--closer to the far end of the row, where a lady and her husband are seated by the aisle. Finally, there is just one seat between your group and the lady. Needing just one more seat to fit your whole group, you ask the lady if she is saving it, or if you can use that last seat. She replies: "Um, my purse is on it." (In an unkind tone of voice.)
(Please leave answer in comments) A--You assume she is saving the seat for someone (even though no one showed up.) B--Smile big and ask if her purse bought a ticket C--Decide to just put up the arm rests and squish the extra person into the seats D--Stare at her in shocked annoyance, then procede to throw popcorn at them throughout the film. E--Other (explained in comments)