Provincial name and city name pull selection box linkage

In the Drop-down form, the first list box has a,b,c three options, and when the user selects a, the second list box displays a1,a2,a3 three options (if the user selects B in the first list box, the second list box displays B1,B2,B3, and so on), when the user selects A1 in the Second list box, The third list box displays A11,a12,a13. How do I achieve this?
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Give you one, and so on.
<script language= "JavaScript" >
<!--
var Onecount;
onecount=0;

subcat = new Array ();
Subcat[0] = new Array ("Xuhui District", "01", "001");
SUBCAT[1] = new Array ("Jiading", "01", "002");
SUBCAT[2] = new Array ("Huangpu", "01", "003");
SUBCAT[3] = new Array ("Nanchang", "02", "004");
SUBCAT[4] = new Array ("Jiujiang", "02", "005");
SUBCAT[5] = new Array ("Shangrao", "02", "006");

onecount=6;

function Changelocation (LocationID)
{
document.myform.smalllocation.length = 0;

var Locationid=locationid;
var i;
Document.myform.smalllocation.options[0] = new Option (' = = = = = = ALL Regions = = = = '];
For (I=0;i < onecount; i++)
{
if (subcat[i][1] = = LocationID)
{
Document.myform.smalllocation.options[document.myform.smalllocation.length] = new Option (subcat[i][0], subcat[i][2 ]);
}
}

}

-->
</script>

<form name= "MyForm" method= "POST" >
<select name= "Biglocation" >
<option value= "selected>" Shanghai </option>
<option value= > Jiangxi </option>
</select>
<select name= "Smalllocation" >
<option selected Value= "" >== All areas ==</option>
</select>
</form>
---------------------------------------------------------------

<script language=javascript>
<!--
ct = new Array ();
Ct[0] = "none";
Municipality--Beijing 1,1
CT[1] = "Beijing";
Municipality--Shanghai 2,1
CT[2] = "Shanghai";
Municipality--Tianjin 3,1
CT[3] = "Tianjin";
Municipality--Chongqing 4,1
CT[4] = "Chongqing";
Major cities in Hebei province 5,10
CT[5] = "Shijiazhuang";
CT[6] = "Baoding";
CT[7] = "Qinhuangdao";
CT[8] = "Handan";
CT[9] = "Chengde";
CT[10] = "Langfang";
CT[11] = "Zhangjiakou";
CT[12] = "Cangzhou";
CT[13] = "Xingtai";
CT[14] = "Tang";
15,10, a major city in Shanxi province
CT[15] = "Taiyuan";
CT[16] = "Changzhi";
CT[17] = "Datong";
CT[18] = "Linfen";
CT[19] = "Shanyin";
CT[20] = "Xinzhou";
CT[21] = "Yangquan";
CT[22] = "Yuanping";
CT[23] = "Zezhou";
CT[24] = "Taigu";
Major cities in Inner Mongolia Autonomous Region 25,10
CT[25] = "Hohhot";
CT[26] = "Baotou";
CT[27] = "Wuhai";
CT[28] = "Linhe";
CT[29] = "Dongsheng";
CT[30] = "Chifeng";
CT[31] = "Tongliao";
CT[32] = "Xilin Hot";
CT[33] = "Ulanhot";
CT[34] = "Hailaer";
The main city of Liaoning province "; 35,10
CT[35] = "Shenyang";
CT[36] = "Dalian";
CT[37] = "Anshan";
CT[38] = "Fushun";
CT[39] = "Benxi";
CT[40] = "Dandong";
CT[41] = "Jinzhou";
CT[42] = "Yingkou";
CT[43] = "Liaoyang";
CT[44] = "Tieling";
The main city of Jilin Province; 45,9
CT[45] = "Changchun";
CT[46] = "Baicheng";
CT[47] = "Songyuan";
CT[48] = "Siping";
CT[49] = "Jilin";
CT[50] = "Liaoyuan";
CT[51] = "BSS";
CT[52] = "Tonghua";
CT[53] = "Yanbian";
A major city in Heilongjiang province; 54,10
CT[54] = "Harbin";
CT[55] = "Daqing";
CT[56] = "Heihe";
CT[57] = "Jixi";
CT[58] = "Jiamusi";
CT[59] = "Mudanjiang";
CT[60] = "Nenjiang";
CT[61] = "Qiqihar";
CT[62] = "Suihua";
CT[63] = "Yichun";
A major city in Jiangsu Province; 64,12
CT[64] = "Nanjing";
CT[65] = "Suzhou";
CT[66] = "Wuxi";
CT[67] = "Changzhou";
CT[68] = "Zhenjiang";
CT[69] = "Yangzhou";
CT[70] = "Nantong";
CT[71] = "Xuzhou";
CT[72] = "Lianyungang";
CT[73] = "Huaiyin";
CT[74] = "Taizhou";
CT[75] = "Suqian";
A major city in Zhejiang province; 76,10
CT[76] = "Hangzhou";
CT[77] = "Jinhua";
CT[78] = "Ningbo";
CT[79] = "Wenzhou";
CT[80] = "Huzhou";
CT[81] = "Shaoxing";
CT[82] = "Jiaxing";
CT[83] = "Taizhou";
CT[84] = "Zhoushan";
CT[85] = "Lishui";
Major city of Anhui Province; 86,10
CT[86] = "Hefei";
CT[87] = "Wuhu";
CT[88] = "Bengbu";
CT[89] = "Fuyang";
CT[90] = "Huainan";
CT[91] = "Huaibei";
CT[92] = "Liu'an";
CT[93] = "Ma On Shan";
CT[94] = "Tongling";
CT[95] = "Anqing";
A major city in Fujian province; 96,9
CT[96] = "Fuzhou";
CT[97] = "Xiamen";
CT[98] = "Quanzhou";
CT[99] = "Zhangzhou";
CT[100] = "Longyan";
CT[101] = "Nanping";
CT[102] = "Ningde";
CT[103] = "Putian";
CT[104] = "sanming";


The main city of Jiangxi Province; 105,10
CT[105] = "Nanchang";
CT[106] = "Jiujiang";
CT[107] = "Xinyu";
CT[108] = "Ganzhou";
CT[109] = "Jingdezhen";
CT[110] = "Shangrao";
CT[111] = "Fuzhou";
CT[112] = "Yichun";
CT[113] = "Ji '";
CT[114] = "Yingtan";
A major city in Shandong province; 115,10
CT[115] = "Jinan";
CT[116] = "Qingdao";
CT[117] = "Zibo";
CT[118] = "Texas";
CT[119] = "Yantai";
CT[120] = "Weifang";
CT[121] = "Jining";
CT[122] = "Zaozhuang";
CT[123] = "Sunshine";
CT[124] = "Laiwu";
A major city in Henan province; 125,10
CT[125] = "Luoyang";
CT[126] = "Jiaozuo";
CT[127] = "Kaifeng";
CT[128] = "Nanyang";
CT[129] = "Pingdingshan";
CT[130] = "Sanmenxia";
CT[131] = "Shangqiu";
CT[132] = "Xuchang";
CT[133] = "Zhengzhou";
CT[134] = "Zhumadian";
CT[134] = "Hebi";
A major city in Hubei province; 135,10
CT[135] = "Wuhan";
CT[136] = "Yichang";
CT[137] = "Shiyan";
CT[138] = "xianning";
CT[139] = "Yellowstone";
CT[140] = "Huanggang";
CT[141] = "Xiangyang";
CT[142] = "Yicheng";
CT[143] = "Xiangfan";
CT[144] = "Jingzhou";
A major city in Hunan province; 145,10
CT[145] = "Changsha";
CT[146] = "Zhuzhou";
CT[147] = "Xiangtan";
CT[148] = "Changde";
CT[149] = "Hengyang";
CT[150] = "Yueyang";
CT[151] = "Huaihua";
CT[152] = "Yongzhou";
CT[153] = "autonomous continent";
CT[154] = "Zhangjiajie";
A major city in Guangdong province; 155,8
CT[155] = "Guangzhou";
CT[156] = "Shenzhen";
ct[157] = "Foshan";
CT[158] = "Zhanjiang";
CT[159] = "Shantou";
CT[160] = "Zhuhai";
CT[161] = "Conghua";
CT[162] = "Thai letter";
Major city of Guangxi Zhuang Autonomous Region; 163,10
CT[163] = "Nanning";
CT[164] = "Baise";
CT[165] = "Beihai";
CT[166] = "Beiliu";
CT[167] = "Fangchenggang";
CT[168] = "Guilin";
CT[169] = "Liuzhou";
CT[170] = "Luzhai";
CT[171] = "Luchuan";
CT[172] = "Pingguo";
A major city of Hainan Province; 173,10.
CT[173] = "Haikou";
CT[174] = "Qiongshan";
CT[175] = "Lingao";
CT[176] = "Wenchang";
CT[177] = "Changjiang";
CT[178] = "White Sands";
CT[179] = "Qiongzhong";
CT[180] = "Sanya";
CT[181] = "Lingshui";
CT[182] = "Qionghai";