Reaction of BeO with ammonia and hydrogen fluoride gives 'A' which on thermal decomposition gives BeF$$_2$$ and NH$$_4$$F. What is 'A'?

A

H$$_3$$NBeF$$_3$$

B

(NH$$_4$$)$$_2$$BeF$$_4$$

C

NH$$_4$$BeF$$_3$$

D

(NH$$_4$$)$$_2$$Be$$_2$$F$$_6$$