Five “Infeasibility” is really a noun and “unfeasible”, as nan grey notes, can be an adjective herisson KeldericKelderic 47211 gold badge44 silver badges88 bronze badges 10 You should supply proof. Long ago, one of my teachers advocated "none is", so I use that (other than often I don't). Ngram suggests https://sethoxfmu.ourabilitywiki.com/10323628/detailed_notes_on_45_gap_ammo_price