covariance calculation

I don’t get why it is implied here that this is a sample? Well- I guess I do in the sense that we can assume these stock’s have more returns we can’t see in the table here. But I did this calculation of variance using n=4 not n=3…I guess I’m curious how the exam might phrase a question like this? WIll they use the word ‘sample’ to make it easy, and if not, should I assume any table of stock retunr over a period must be a sample?

An analyst observes the following return behavior between stocks X and Y.

Time Period X’s Return Y’s Return 1 7 5 2 9 8 3 10 11 4 10 8

What is the covariance of returns between stocks X and Y?

I think it will be specified.