Balancing equations
_ N H4 F+_AlCl3 -_ NH4Cl + _ AlF3​

it's c (engineer's do their work with mechanical stuff not on any living being)

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.