1.d4 Nf6 2.c4 e6 3.g3 d5 4.Bg2 dxc4 5.Nf3 Be7. What is the ECO code of
this opening?

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How to find the answer: Search for ["1.d4 Nf6 2.c4 e6 3.g3 d5 4.Bg2 dxc4 5.Nf3 Be7"] to find this exact sequence of terms. This is the chess opening known as Catalan. Search [ECO code] to find that it stands for Encyclopedia of Chess Openings. Then, [ECO code for Catalan] find that the Catalan and its variants have ECO codes E01 to E09, but this particular opening is considered the classic, E05.