Select the best word or phrase that completes each sentence:…

Select the best word or phrase that completes each sentence: (addition, allylic substitution, retrosynthetic analysis, green chemistry, water chemistry, heterolysis, homolysis, increases, saturated, inductive effects, inhibitor, synthetic, initiator, ionic, less, more, route, paired, radical resonance, elimination, bromination, unsaturated, unpaired,  alkanes, alkenes, alkynes, functional group)   In a(an) ……………………….,  the last step of synthetic route is first established and the steps are determined working backward from the product. Complete word/phrase and correct spelling (verbatim) is required. No partial credit.