How do you differentiate between internal and external root resorption?

The classic method to differentiate internal vs. external root resorption is CHANGING RADIOGRAPHIC ANGULATION (Clark's rule / SLOB): INTERNAL resorption = the defect MOVES WITH the cone (stays centered in the root canal) when angulation is changed because it's within the canal. EXTERNAL resorption = the defect MOVES AWAY from the cone (moves buccally or lingually) when angulation changes because it's on the external root surface. Pulp vitality is also helpful (internal = vital initially; external = non-vital if periapical). A is the radiographic differentiation method. A is correct.