C stands for out-of-order volley. If two balls are drawn, all the events are C 10 (lower right) 2 (upper right) =( 10*9)/(2* 1). If it is three balls, it is C 103 = (18).

A refers to the sequential drawing of the ball. For example, two balls are drawn with different colors, that is, A 10 (lower right) 2 (upper right) = 10 * 9. If you score three goals, it will be A 10 3 = 10 * 9 * 8 and so on.

I hope you can understand.