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Adjectives cannot be plural in English. They also cannot be singular because adjectives are not marked for number at all in English. Things like these students and those students use plural demonstrative determiners not plural adjectives, and things like professors emeriti or professors emeritae (not to mention appellations contrôlées, nouveaux arrivés, musicas norteñas, tortillas españolas, uomini universali) all use non-English plural adjectives, which doesn't count.