title: how to use multiple FormArray in angular5
body: <p>I have such form. And I want to create dynamic multiple FormArray when I change Category dropdown. These FormArrays are inside *ngFor in html. How Can I fix it? Thanks in advance!  </p>
  
  <p><a href="https://i.stack.imgur.com/xlYNb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xlYNb.png" alt="enter image description here"></a></p>
  
  <p>I have already seen this
  <a href="https://alligator.io/angular/reactive-forms-formarray-dynamic-fields/#importing-formarray" rel="nofollow noreferrer">https://alligator.io/angular/reactive-forms-formarray-dynamic-fields/#importing-formarray</a></p>
  
  <p>and it works. But I can't use for multiple. </p>