Passage: Formerly Bang Bon was a tambon of amphoe Bang Khun Thian in Thonburi Province, prior to the merger of Thonburi and Phra Nakhon into a single province, after which it was a sub-district of Bang Khun Thian District.

Question: Which areas were joined together to form just one province?
Thonburi and Phra Nakhon