I have to say I think it's quite possible and comical that the professor interpreted your work on two of the three questions as additional evidence that you did know it should be an "and", since I would guess relatively few students would attempt any more than _one_ permutation if the assignment only asked for one.
But what did the other students the class do? If almost everyone in the class did only one, the professor would have to give almost the entire class 33% for a mistake he made.
Looking back, I should have asked around. The job scheduler was one of several options so not everybody picked it. Still, somebody else must have.
The professor actually did relent and gave me a good (not perfect, but good) grade. Then he judged me extremely harshly on the final exam so I ended up with a B in the course anyway.
Damned if you do, damned if you don't.