There is an apple on a desk. Your friend who can't see it asks, "are there any apples on the desk?"
What is the short answer to this question?
"Yes, there is" or "Yes, there are."
Or vise versa, I mean there are some apples on the desk and your friend asks, "is there an apple on the desk?"
We should answer in short form based on the structure of the question and then explain it in the long answer? Or we can simply answer based on what we know?