Why is image classification competition the way to choose five candidates? not 4, not 3, especially not 1?

Question

Why Image classification competition ask 5 candidates answer?

I mean, if Image is about Orange, then (Apple, Car, Airplane, Book, Orange) is right answer.

But if the competition asked 3 candidates, and this machine only answer first 3 -(Apple, Car, Airplane), then it wrong.

So, 5 candidates answer is a lot easier way then 3 candidates. and 3 is ~ 1.

Why 5 candidate answer - of all numbers especially 1- ask?

I spent half hour to research about it.

Answer 1

Firstly, the classifier is not yet very accurate. It can encourage researchers. Secondly, the objects in the picture are not one.