Chess Opening Theory/1. e4/1...c6/2. d4/2...d5/3. exd5/3...cxd5/4. Bd3/4...Nc6

= Caro-Kann Exchange Variation = c3 is probably the only sound response. Nf3 has a terrible record, while Ne2, or tying up the dark bishop with Be3, both seem unsound.

Theory table
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1.e4 c6 2.d4 d5 3.exd5 cxd5 4. Bd3 Nc6