Answer: A

Reasoning: After ingestion, fluoride is rapidly absorbed from the gastrointestinal tract; only a minority is taken up in the stomach, while the great majority (≈75-90 %) is absorbed further along the small intestine, making the intestine the principal site of systemic uptake in children and adults alike [Nopakun 1989, PMID 2585131; Buzalaf & Whitford 2011, PMID 21701189].