When white opens 1.d4, 2.c4, sometimes black moves at some point d5 and then c5, resulting in a square of 4 pawns. This is confusing to me. Depending on the exact opening, the most played move for white is either moving a knight (if it hasn't already been moved), take with the c-pawn, take with the d-pawn, or e3. Can somebody shed some light on this to me?
Some examples with the most usual response by white:
[FEN ""] 1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5
[FEN ""] 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 c5 5.e3
[FEN ""] 1. d4 d5 2.c4 c5 3.Nf3
Grunfeld 3 knights
[FEN ""] 1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.Nf3 Bg7 5.Bf4 c5 6.dxc5