What does xd mean when you put in the end of a sentence

Have you (singular) ever thought about being an archaeologist? i have! think what archaeologists give us! because of their efforts we learn from a piece of pottery that there were shops from which a roman could "special order" a piece of statuary. they tell you (singular) whether or not a statue is from the archaic or hellenistic periods. if you and those friends of yours (singular) are lucky, and few of you (plural) will be, you (plural) may work with a team on a dig. they will work with you (plural), train you (plural) and give you (plural) fascinating experiences from which to learn and to remember with joy in later life. how i wish the opportunity had been given me when i was your age! you (plural) (2 points) select one: a. nobis b. nostrum c. vobis d. vestrum