题目某品牌电脑原价nnn元,降价20%20\%20%后又降低mmm元,该电脑现价（单位:元）为（ ）元A.45(n−m)\frac{4}{5}(n-m)54(n−m)B.(54n−m)(\frac{5}{4}n-m)(45n−m)C.(45n−m)(\frac{4}{5}n-m)(54n−m)D.(54m−n)(\frac{5}{4}m-n)(45m−n)知识点：列代数式章节：未标注
