Button display in a form array based on a formcontrol

I've a form array named Questions inside which there are FormControls like QuestionText, AnswerType and AnswerList(this is also a form array). I've seperate buttons for adding questions as well as for adding answers to this Answerlist. Everything works fine except for the (add ans) button. I don't want this button to be visible all the time. If the user selects the AnswerType as single choice, the button should be displayed. If the user selects the AnswerType as Boolean, the button should not be displayed.

<div formArrayName="Questions" >
<div 
  *ngFor="let filter of QuestionsFormArray.controls; let j = index;">
  <div [formGroupName]="j">
    <mat-form-field>
      <input matInput formControlName="displayText" placeholder="Question Text">
    </mat-form-field>
    <mat-form-field>
        <select formControlName="AnswerType" placeholder="Select AnswerType" (change)="changeanswertype($event,j)" matNativeControl required>
          <option *ngFor="let item of AnswerType" [value]="item.type">
            {{item.type}}</option>
        </select>
      </mat-form-field>
      <div>
        <button *ngIf="filter['controls'].AnswerType!='Boolean'" class="btn btn-select rightfloat" (click)="AddAnswer(filter.controls.Answers, j)">Add ans</button>
      </div> &nbsp;&nbsp;&nbsp;
      <mat-icon class="deleteicon"
      (click)="removeFilterFromQuestionsFormArray(j)">
      delete
    </mat-icon>
  <div formArrayName="Answers" >
    <div *ngFor="let ans of filter['controls'].Answers['controls']; let k = index;">
        <div [formGroupName]="k" >
    <mat-form-field>
        <input 
          matInput 
          formControlName="text" 
          placeholder="AnswerText">
      </mat-form-field>
      <button type="button" class="close" aria-label="Close" (click)="removeanswer(j,k)">
          <span aria-hidden="true">&times;</span>
        </button>
      </div>
    </div>
  </div>
  </div>
</div>