In my AP European History class in high school, we had a 25 question quiz the next day every time we were assigned a chapter to read in the textbook (maybe once every week or two). They were intentionally difficult to try to encourage us to read closely, often asking about specific details that were only mentioned once in the chapter, and they were heavily curved, so getting even around half of the questions right generally would get you a decent grade. The teacher had a habit of making the last choice of each question be a combination of two of the earlier choices (e.g. "both A and C"), and these often ended up being the right answer to the point where one of my classmates came up with the idea of choosing the last choice in every single question to try to get an easy passing score. Unfortunately for him, he was a bit too proud of this idea and talked about it enough beforehand that the teacher caught wind of it and purposely made none of the answers on the next quiz have the last choice be correct. Not sure how well that would go over in the Navy, but I'd certainly still be a bit worried that the exam writer might know the rule of thumb people use and have a bit of fun at my expense!

But why C, and not A or B or D?

I've heard there's a tendency to put the "right" answer in the middle, especially if the answers to a question are numerical.

Or maybe it's just that you need to pick something, so why not C?

I heard it every time I took the SAT.