Diagram 1, White invaded 3-3. Where should black move next? A or B?
Diagram 2, If Black chose B, the result doesn’t benefit the triangle-marked stone. Not Efficient.
Diagram 3, Black moves at A is correct, if White extends at move-2, White will ended up living in gote. Which is not the best result in most circumstances because if black is thick outside, Black’s move-7 will be played at move-8 to kill White.
Diagram 4, White kosumi move-1 is correct in this situation. Black next moves ranged from A, B or C.
Diagram 5, Regardless of answer A or B, the result is the same for White.
Diagram 6, Black answers at move-1 because White surely lives thus Black prevents White to proceed further to left.
Diagram 7, If Black move-7 from Diagram 6 has decided to drop at move-1 here, White can easily escaped and destroyed Black’s thickness.
Diagram 8, This is a standard joseki, where you can expect the invasion to occur.
Diagram 9, after the exchange of Black from move-1 to move-5, White invades at move-6 and you will expect the same result as Diagram 6.