In the following opening:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4
There is a well-known trap if Black tries to defend the c file pawn with:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 c6
However I got the following played against me:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7
How should White respond?