Answer c3 with ...d5. After Qxd5 lines, White gets an isolated d-pawn; blockade on d5. Against e5, push ...d4 for space. Also reached by 8.Nc3 Qd6 9.O-O Nc6, and by 2.c3 d5 3.exd5 Qxd5 4.d4 Nf6 5.Nf3 e6 (also for lines 17 and 18). Merged lines 16, 46, 47.