R² such that ∀(x1, y1), (x2, y2) ∈ R², (x1, y1) ∼ (x2, y2) ⇒ x2 - x1 = 3(y2 - y1). What type of relation is described here?
a) Linear relation
b) Quadratic relation
c) Cubic relation
d) Exponential relation