Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Bxc6/4...dxc6

= Ruy Lopez, Exchange Variation = In order to attack the e5 pawn, White must first protect themself against the Qd4 threat (everyone must be aware that /5. Nxe5/ only will give white disadvantage.).
 * /5. Nc3/ is an option but probably not the best, and anyway not the most active.
 * /5. O-O/ is a much more subtle way to defend against the pawn capture, as after Nxe5 Qd4 Nf3 Qxe4, White can capture the queen with Re1 (the Black Queen is pinned because it protects the king).
 * /5. d4/ is a way to break the center and solve matters in blood. But this variation does not seem to get good results for White.

Theory table
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 * 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4. Bxc6 dxc6