Reaction of \( \mathrm{BeO} \) with ammonia and hydrogen fluoride gives A which on thermal decomposition gives \( \mathrm{BeF_2} \) and \( \mathrm{NH_4F} \). What is 'A'?
\( (\mathrm{NH_4})\mathrm{Be_2F_5} \)
\( (\mathrm{NH_4})\mathrm{BeF_3} \)
\( (\mathrm{NH_4})_2\mathrm{BeF_4} \)
\( \mathrm{H_3NBeF_3} \)