Adds up to 180 degrees. For two angles to be non-supplementary the must sum to anything other than 180 degrees, to be non-adjacent they can't share a common side (Ray)
Number of proper subsets = 2^number of elements- 1
= 2^n - 1
No of elements = 4
2^4-1 = 16-1 = 15
C because I did the math on an app so it’s right
