select 清除默认样式

 select{
    -webkit-user-select:none;
    -moz-user-select: none;
    user-select: none;
　　

    border: 0 none;
    border-radius: 0;
    outline: none;
    background: none;
    appearance: none;
    -webkit-appearance: none;
    -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
  }

  select:focus{
    border: 0px;
    box-shadow: 0 0 0px 0px #fff;
  }

 用padding 来设置使文字居中