How Should the Therapist Determine the Depth of Insertion of an Endotracheal

How should the therapist determine the depth of insertion of an endotracheal tube marked with three rings in an infant during the intubation procedure?

A) Just when the Murphy eye clears the vocal cords and enters the trachea
B) At the location where the second double-ring mark just passes the glottis
C) At the point where the first heavy black line just moves beyond the glottis
D) Just after the distal third of the tube passes into the trachea past the glottis