"what source should contracting officers use as their primary source of vendor information"

The correct answer to this open question is the following.

The source should contracting officers use as their primary source of the vendor information is the Central Contractor Registration (CCR), now called System for Award Management (SAM).

CCR served until July 2012. This was the place the government used to select contractors for public projects. After July 2012, the government replaced the CCR with the aforementioned System for Award Management (SAM). COntractors that already had their information in the old database, automatically migrated to the new database.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.