There are many questions with only one answer. Or at least, one sensible answer. If someone asks you "Are you dead?", the only possible answer you can give is "no". "Does my bum look big in this?" "Of course not, dear, it looks lovely". And if you say "Is there some kind of problem, officer?" you know there's only going to be one answer.
I get asked questions with only one answer in maths all the time. I used to roll my eyes, but these days I tell my student that it's a question with only one answer. Here are some of my favourites:
Yes. If you have something you can sensibly draw a picture of, you should draw it. It doesn't have to be a Rembrandt, just something to give you a visual stimulus. Your brain works better with pictures than it does with numbers.
No. If you've got to ask, then no, it's not big enough. There aren't many pictures in maths that can't be improved by enlarging them.
No. I used to work in a printing press. The amount of paper they'd waste in an evening would dwarf all the paper I could possibly use in a lifetime. It's not a waste of paper to turn over and give yourself plenty of room, especially if it saves you from having to do it all over again.
Yes. If I'm writing it down, it's probably important.
What do you think the best way to find out might be?
No, you can't. You can have some paper, but I don't want it back.
D'uh. I can't think of any circumstance (except, possibly, when you're in a multiple choice exam) where showing your work is a bad idea. If it's for homework or your own notes, the more detail you can give about how you get to the answer, the more you'll thank yourself when you come to revise.
How could you tell?
What other questions can you think of that have only one answer?