Puzzle of the week #323

Chess Diagram: 

[Event "Puzzle #323"][Date "2017.09.30"][Result "?-?"]1. d4 d6 2. Nf3 Bg4 3. h3 Bxf3 4. gxf3 Nf6 5. Nd2 e6 6. b3 Nd5 7. c4

This is the first puzzle for the new level 2 class 2017-2018. Please replay the moves leading to this position and answer to the following questions:
a) Which is the weakest pawn for white and black at the beginning of the game?
b) White's last move 7. c4 ... is attacking black's Knight. What should black do here?

Total available points for this puzzle is 12. The answers will be published next time together with puzzle #324.

Puzzle #322 solution:
Puzzle by GM Susan Polgar, 2017. Black wins with a swift and decisive attack on white's King.

[Event "Puzzle #322"][Date "2017.06.29"][Result "0-1"][SetUp "1"][FEN "r5k1/p1QN3p/P5pB/5p2/1P2nP2/3q3P/3n2PK/R2R4 b - - 0 1"]1... Nf3+ (1... Qg3+ 2. Kh1 (2. Kg1 Nf3+ 3. Kf1 (3. Kh1 Qh2#) 3... Qf2#) 2... Nf2+ 3. Kg1 Nxh3+ 4. Kh1 Nf2+ 5. Kg1 {it is only a draw}) 2. gxf3 Qe2+ 3. Kh1 (3. Kg1 Qf2+ 4. Kh1 Ng3#) 3... Qxf3+ 4. Kh2 Qg3+ 5. Kh1 Nf2# 0-1


The f2/ f7 weak spot