i^1=i
i^2=-1
i^3=-1*i
1^4=-1*-1=1
this repeats itself over and over again:
i^5=-1*-1*i=i
..
31/4=6 with a remainder of 3, so for i^31 it repeated 6 times and then it is the third entry of the table:
so we know i^31=i^3 -> look it up above =-1*i=-i=3rd option
