If there is 1 child, there can be 0 interactions.
If there are 2 children, there can be 3 interactions.
If there are 3 children, there can be 9 interactions.
If there are 4 children, there can be 18 interactions.


How many interactions will there be for 9 children?

a. 135
b. 84
c. 93
d. 108