"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.

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.