The four center squares, d4, d5, e4, e5, are the most valuable real estate on the board. Pieces posted there control more squares in every direction than pieces on the rim, and whoever dominates the center usually dictates what happens everywhere else.
This is why the first few moves of almost every strong opening either occupy the center with a pawn or develop a piece that pressures it. A move that doesn't do either of those things, in the opening, is usually a wasted one.
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a
White has played e4 and Nf3, both fighting for the center. Black to move.
Which move both defends the e5 pawn and develops toward the center?
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a