 White cannot really prevent this plan directly, so he must immediately generate some counterplay in the center with 9. e4.It is possible to play also 9. a4, but the move 9.0-0 is a strategic mistake as it allows black to freely push his c-pawn which equalizes the game immediately.Black will respond with 9...c5, after which White has the choice between pushing his d-pawn or e-pawn, leading to very different middlegames.