其一

one of the given (options etc) / the first / firstly

his / her / its / their / that / such / it (refers to sth preceding it)

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

to know the first, but not know the second (idiom); only partial information