Create a three-level directory that can return to the previous directory and exit the program at any time

data = { 
    'Zhejiang':{ 
        "Shaoxing":{ 
            "Keqiao":["A","B"], 
            "Shangyu":["C","D"] 
        }, 
        "Hangzhou":{ 
            "Xiaoshan ":["E","F"], 
            "Yuhang":{"G","H"}, 
            "Xiasha":{"X","Y"}, 
        }, 
        "Wenzhou":{}, 
    } , 
    'Shandong':{ 
        "Dezhou":{}, 
        "Qingdao":{}, 
        "Jinan":{} 
    }, 
    'Guangdong':{ 
        "Dongguan":{}, 
        "Changshu":{}, 
        "Foshan" :{},
    },
}
mark=False
while not mark:
    for i in data:
        print (i)
    choice1=input('第一层选择>>')
    if choice1 in data:
        while not mark:
            for i in data[choice1]:
                print (i)
            choice2=input('第二层选择>>')
            if choice2 in data[choice1]:
                while not mark:
                    for i in data[choice1][choice2]:
                        print (i)
                    choice3=input('第三层选择>>')
                    if choice3 in data[choice1][choice2]:
                        while not mark:
                            for i in data[choice1][choice2][choice3]:
                                print (i)
                            choice4=input('第四层选择>>')
                            if choice4=='b':
                                break
                            if choice4=='q':
                                mark=True
                    if choice3=='b':
                        break
                    if choice3=='q':
                        mark=True
            if choice2=='b':
                break
            if choice2=='q':
                mark=True

    if choice1=='b':
        break
    if choice1=='q':
        mark=True