Set default value for <select> in Angular 2

I have found two ways:

1. use <select> within <form>:

<form [formGroup]="accountForm">
  <select formControlName="sex">
    <option value="">Please choose one</option>
    <option value="female">Female</option>
    <option value="male">Male</option>
  </select>
</form>

2. set the selected attribute like this without <form>:

<select formControlName="sex">
    <option value="" [selected]="isSelected"> Please choose one </option>
    <option value="female"> Female </option>
    <option value="male"> Male </option>
</select>

By the way, only <option selected> will not work.