disable select in angular2

I am writing angular2 app. I need all my buttons and select with be disabled and then I need to enable them. I want to do that by using my class field butDisabled but with no luck. I have this code:

@Component({
selector: 'my-app',
template:`
<h1>Disable Enable</h1>
<h2> this is a disabled select </h2>
<select type="number" [(ngModel)]="levelNum" (ngModelChange)="toNumber()" disabled>
<option *ngFor="let level of levels" [ngValue]="level.num">{{level.name}}</option>
</select>
<h2> this is a disabled button </h2>
<button type="button" class="btn btn-default {{butDisabled}}">Disabled</button>

<h2> Why can't I disable the select the same way? </h2>
<select type="number" [(ngModel)]="levelNum" (ngModelChange)="toNumber()" class="{{butDisabled}}">
<option *ngFor="let level of levels" [ngValue]="level.num">{{level.name}}</option>
</select>


`,
})
export class AppComponent {
butDisabled = "disabled";
levelNum:number;
levels:Array<Object> = [
{num: 0, name: "AA"},
{num: 1, name: "BB"}
];

}


I don't understand why I can't use the {{butDisabled}} to disable the select. What Am I doing wrong? Thanks a lot

here is a plunker

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