vue中v-model对select的绑定

1、单选时
<select v-model="selected">
    <option disabled value="">请选择option>
    <option>Aoption>
    <option>Boption>
    <option>Coption>
  select>
  <span>Selected: {{ selected }}span>
data: {
    selected: ''
  }
如果 v-model表达式的value初始值未能匹配任何选项,