2...d5 is Black's best chance to unbalance the position. Black is prepared to recapture on d5 with the queen as White's pawn on c3 prevents its immediate harassment. After 3.exd5 Qxd5 4.d4, Black has several options, the most common being 4...Nf6, 4...Nc6, and 4...e6. Some of White's more interesting responses involve an early Na3. See the below as an example, and compare to Game 21.