题目

涂色原则为:每相邻两块不同色,每块只能涂1色.某市有4个郊县(如图),现备有5种颜色,有___________种不同的涂色方式. 答案：答案:420解析:先涂市区A,有5种选择;再涂B、C,分别有4种和3种选择.涂D时,既可采用第4种色,也可采用第5种色或B县所涂色.如采用第4种色,则E可从第5色与C县所涂色中选1色;如D采用B县所涂色,则E可从第4、5两色及C县所涂色中选1色,故共有5×4×3×+5×4×3×1×=420种涂色方式.So much progress _____ in the past few months that she now has no difficulty in making herself understood in English.A．did she makeB．has she madeC．she has madeD．she made