Complete the letter–mirror pattern BCB, DED, FGF, HIH, ___ by recognizing that each of the three positions advances by +2 letters, retaining the palindromic structure.

Palindromic triplets with uniform +2 steps across positions.

• First letters: B → D → F → H → J (+2).
• Middle letters: C → E → G → I → K (+2).
• Last letters mirror the first: B → D → F → H → J.