OK.
Back in the old days when e-lec-tricity was a new fangled adventure, batteries were two cells connected, one positive, one negative. Thus the A battery and the B battery. When dry cell batteries came about, many old timers, who refused to read instructions, assumed that the new batteries would be the same instead of having a positive and negative on a single battery.
I can attest to the fact that some old timers are hard headed. My father is 85 this month, and there's no telling him anything. If he doesn't know it, he'll make up an answer. Beyond that he doesn't really care.
Anyway, the battery manufacturers decided that A's , C's and D's would work, thus eliminating the attempt by stubborn old men to connect unlike batteries together to fit in a flashlight.
There you have it.
We can all go to be knowing that we have learned another bit of useless trivia that you can use... ok, maybe never, but at least you can think you are smarter than a 5th grader.