vue select二级联动第二级默认选中第一个option值

<p class="inputLine">
  <span>所在区域</span>
  <select name="" v-model="countryName" @change="selectCountry">
  <option :value="item" v-for="(item,index) in area">
  {{item.country}}
  <svg class="icon icon-arrow-bottom" aria-hidden="true">
  <use xlink:href="#icon-arrow-bottom" rel="external nofollow" rel="external nofollow" ></use>
  </svg>
  </option>
  </select>
  <select name="" v-model="cityName">
  <option :value="item" v-for="(item,index) in countryName.city">
  {{item}}
  <svg class="icon icon-arrow-bottom" aria-hidden="true">
  <use xlink:href="#icon-arrow-bottom" rel="external nofollow" rel="external nofollow" ></use>
  </svg>
  </option>
  </select>
 </p>

data
  countryName:{},
  cityName:"请选择城市",
  area:[
   {
   "country":"美国",
   "city":[
    "纽约",
    "洛杉矶",
    "旧金山",
    "西雅图",
    "波士顿",
    "休斯顿",
    "圣地亚哥",
    "芝加哥",
    "其它",
   ]
   },
   {
   "country":"加拿大",
   "city":[
    "温哥华",
    "多伦多",
    "蒙特利尔",
    "其它"
   ]
   },
   {
   "country":"澳大利亚",
   "city":[
    "悉尼",
    "墨尔本",
    "其它"
   ]
   },
   {
   "country":"新加坡",
   "city":[
    "新加坡"
   ]
   },
   /*{
   "country":"中国",
   "city":[
   "北京市",
   ]
   },*/
  ],

selectCountry(value){
  this.cityName=this.countryName.city[0];
  },