Which explanation most accurately accounts for missing vitamin letters (for example, no vitamins labeled F, G, H in many modern lists)?

• A. Gaps reflect a modern international rule that only letters A, B, C, D, E, and K may represent vitamins to harmonize drug nomenclature, and any other previously discovered compounds were absorbed into mineral classification.
• B. Letters were skipped deliberately to match periodic table groupings and because vitamins discovered after the B series were retroactively given letters only if they were fat-soluble, creating gaps for water-soluble molecules.
• C. The missing letters indicate vitamins that are present only in nonhuman species and therefore excluded from the human essential vitamin list, which is why they are not shown on human nutrition charts.
• D. Some letters were assigned historically to substances once thought to be vitamins but later reclassified or found nonessential, so their letter designations were retired, producing gaps in the sequential naming.

Answer: (D)