Here we go again. I'll expand on Edith's comment below about fooling us. Like we're going to remember obscure one off instances in life when we're barely speaking the language yet. I knew you were looking for the singular because it said to use the singular but I couldn't do it. Sometimes selecting the teacher's correct answer is not the correct thing to do. It didn't intuitively make sense to me and DeepL AI translation agrees that it should have been plural. Who is right? The teacher or AI? Maybe both, maybe neither. This is not the way to learn to speak a language. I'm sorry I paid for a year of this website. The teachers here are the ones I would have avoided at all costs in a university atmosphere - 40 years ago. Already left here for a similar reason awhile back. Not sure why I came back.