Correct answer: O-O1...K:b1 2.O-O# 1...Nd1 2.R:a2# 1.Kf1?? zz 1...Nd1 2.R:a2# but 1...K:b1! 1.Rh3?? zz 1...Nd1 2.R:a2# but 1...K:b1! 1.R