The vertices are in three groups:

1. Vertices whose shortest distance from V₀ is known
2. vertices that are adjacent to some known vertex (the frontier vertices)
3. all the other vertices (not known, not in the frontier)

Known: {V₀}; Frontier: { V₁, V₂, V₃}; Other: { V₄, V₅, V₆ }

At each step, one frontier vertex can be added to the known vertices.

Which frontier vertex can become known in this example?