Program BiChromaticCheck;

Const MaxV=400;

Type PVert = ^Vert;
     Vert = Record
       n: Word;
       next: PVert;
     End;

Var fin, fout: Text;
    N, line: Word;
    V, i, a, b: Word;
    C: Array [1..MaxV, 1..MaxV] of Boolean;
    color: Array [1..MaxV+1] of Word;
    first, last, tmp: PVert;
    colora: Byte;
    ok: Boolean;
Begin
  Assign(fin,'bichrom.in'); Reset(fin);
  Assign(fout,'bichrom.out'); Rewrite(fout);
  Readln(fin,N);
  For line:= 1 To N Do Begin
    Read(fin,V);
    FillChar(C, SizeOf(C), False);
    FillChar(color, SizeOf(color), False);
    While (not SeekEoLn(fin)) Do Begin
      Read(fin, a);
      Read(fin, b);
      C[a,b]:= True;
      C[b,a]:= True;
    End;
    Readln(fin);
    ok:= True;
    b:= 1;
    Repeat
      While color[b]<>0 Do Inc(b);
      If b>V Then Break;
      color[b]:= 1;
      New(first);
      first^.n:= b;
      first^.next:= Nil;
      last:= first;
      While first<>Nil Do Begin
        a:= first^.n;
        colora:= color[a];
        For i:= 1 To V Do Begin
          If C[a,i] Then Begin
            If color[i]=0 Then Begin
              color[i]:= 3-colora;
              New(last^.next);
              last:= last^.next;
              last^.next:= Nil;
              last^.n:= i;
            End
            Else Begin
              If color[i]=colora Then Begin
                ok:= False;
                While first<>Nil Do Begin
                  tmp:= first;
                  first:= first^.next;
                  Dispose(tmp);
                  b:= V+1;
                  Break;
                End;
              End;
            End;
          End;
        End;
        If first<>Nil Then Begin
          tmp:= first;
          first:= first^.next;
          Dispose(tmp);
        End;
      End;
    Until False;
    If ok Then Writeln(fout,'YES')
          Else Writeln(fout,'NO');
  End;
  Close(fin);
  Close(fout);
End.