Or there's a curve so you fit each letter to 10% of participants. Generally it's a bell curve though so most people end up in C, since that's "average" or "satisfactory".

I had a few classes in college, where in a very 1984-esque fashion, failing (<60% correct) was passing. You just had to fail less than everyone else.

That was the quick and easy answer. Testing that doesn't expect mastery of the material will, of course, work differently.

C means satisfactory. B means above average. A means exceptional.

The expectation is a 70%. It is expected that the average student will get almost 1/3 wrong.

However, recent grade inflation has skewed the original meaning of these definitions