FacetFilters filter with OR option in .Net

I am trying to filter my data on basis of OR condition in .Net. its working very fine with one condition but if i provide multiple options it work like AND.

Here is my code that is working as AND condition but i NEED THIS to work as OR.

                            JArray j = new JArray();
                            j.Add(String.Format(query, category));
                            j.Add(new string[] { "causeid:6", "causeid:7", "causeid:8" });
                             q.SetFacetFilters(j);

Below code work fine
JArray j = new JArray();
j.Add(String.Format(query, category));
j.Add(new string { “causeid:6”});
q.SetFacetFilters(j);