<prompt> Which of the following OPV vials are usable?Only 1Only 1,2Only 1,2, 3Only 3, 4  <response>: b Only 1, 2Rules for VVM use in India Rule 1: If the inner square is lighter than the outer circle, the vaccine may be used Vials 1, 2 Rule 2: If the inner square is the same colour as, or darker than, the outer circle, the vaccine must not be used Vials 3,4. 