Two frames move with 3-velocities v₁ and v₂ with respect to a fixed inertial observer. The relative velocity v satisfies:
a. v = v₁ + v₂
b. v = v₁ - v₂
c. v = (v₁ + v₂) / (1 + (v₁v₂ / c²))
d. v = (v₁ - v₂) / (1 - (v₁v₂ / c²))