<prompt> In which stage, corneal ulcer involving 12 of Corneal surface seen in WHO classification of vitamin A deficiencyStage 2Stage 3AStage 3BStage 4  <response>: WHO classification of Vitamin A deficiency:I Primary changesX1A Conjunctival xerosisX1B Bitots spotsX2 Corneal xerosisX3A Corneal ulcer 13 rd of corneal surfaceX3B Corneal ulcer 13rd of corneal surface.II Secondary changesXN Night blindnessXF Xeropthalmic fundusXS Xerophthalmia - related corneal Scar 