C is the answer because there is no variable for X which without it would make a horizontal line.
Yes the relation is a function no number repeat
Circumerence=2pir or dpi were r=radius and d=diameter
11pi=2pir
divid eobt sides by pi
11=2r
dvide 2
5.5=r
11pi=dpi
divide both sides by pi
11=d
diameter=11in
radius=5.5 in