Yalta is a rather insignificant holiday resort on the shores of the Black Sea, not the capital (which is Kiyev, btw), right?
I interprete it like that: it needs to be a conference where all the big three (the USSR, the USA and the UK) took part, and it needs to be in a capital. The Soviet Union wasn't there in Washington, DC, USA; Casablanca (Morocco) is not the capital, and Teheran (Iran) was after the conference of....dumdumdum...Moscow!
Russia