Russia says Trump did not give them classified information but... how do they know that?

This tweet from Associated Press looked pretty strange to me:

BREAKING: Russian Foreign Ministry spokeswoman denies reports that Trump revealed classified information to senior officials.

— The Associated Press (@AP) May 16, 2017

Oh yeah? And how does Russia know the information Trump gave them is NOT classifed?

The comment suggests two things:

A) Either Russia is saying the information was not classifed because they already know America's classified information (and thus they just revealed what know US classified information).

B) Trump called the Russians to tell them what he told them wasn't classified information. And Russia, of course, believed it. Right...

Either way, that suggests that yes, the information Trump gave to the Russians WAS classified.