title: Angular 6 Reactive Form - Select options: disable previously selected options
body: <p>I have 3 dropdown menus that must have unique values (I am assigning network adapters here), so if one nic is selected in dropdown 1, it must be disabled in dropdowns 2 and 3. 
  I have found <a href="https://stackoverflow.com/a/27596918/10092371">this answer</a> and a few other's, but I can't get them to work. </p>
  
  <p>component.ts</p>
  
  <pre><code>nicAdapters: any[] = ['nic0','nic1','nic2','nic3','nic4','nic5','nic6','nic7','nic8','nic9','nic10']
  
  
     this.inputForm = this.fb.group({
      upLinks: this.fb.group ({
           NumberUplinks: ['2'],
              uplinksMgmt: this.fb.group ({
                  uplink1: ['nic0'],
                 uplink2: ['nic1'],
                 uplink3: ['nic3'],
              })
          })
  })
  </code></pre>
  
  <p>component.html</p>
  
  <pre><code>&lt;div class="select" formGroupName="uplinksMgmt"&gt;
      &lt;select formControlName="uplink1" id="uplink1Id" class="selectBox"&gt;
          &lt;option *ngFor="let uplink1x of nicAdapters" [ngValue]="uplink1x"&gt;{{uplink1x}}&lt;/option&gt;
      &lt;/select&gt;
  &lt;/div&gt;
  &lt;div class="select" formGroupName="uplinksMgmt"&gt;
       &lt;select formControlName="uplink2" id="uplink2Id" class="selectBox"&gt;
         &lt;option *ngFor="let uplink2x of nicAdapters" [ngValue]="uplink2x"&gt;{{uplink2x}}&lt;/option&gt;
      &lt;/select&gt;
  &lt;/div&gt;
  &lt;div class="select" formGroupName="uplinksMgmt"&gt;
      &lt;select formControlName="uplink3" id="uplink3Id" class="selectBox"&gt;
          &lt;option *ngFor="let uplink3x of nicAdapters" [ngValue]="uplink3x" {{uplink3x}}&lt;/option&gt;
      &lt;/select&gt;
  &lt;/div&gt;
  </code></pre>