I'm going over old test results, and for whatever reason I just can't get this one into my head.

Given the proposition

[it is, of course]

determine whether it is a tautology, and, if it is a tautology, write the valid argument determined by it.

In other words, figure out what rule of inference can be derived from this argument. I believe it ismodus tollens, but am not sure, and just can't see what method I would use to go about this. Any ideas?

Thanks to all the good people out there!