What does XD mean? I keep seeing people say it and I dont know what it means

Two technicians are discussing the common u-joint. technician a says its input and output speeds should be equal. technician b says that it normally has two yokes. which technician is correct?

I'm taking a class on how to make a movie, and it offers some apps that would be in the process. the thing is, i don't have any of those ha. if you have any tips on some apps i could use, that would be awesome. i don't have an iphone, so don't suggest any apps like imovie. i know that this is a weird question, but it would be super for me. : )

What is the best assassins creed game?