How can you direct a drag and drop to go to a specific textbox?

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C#

There are two listboxes and two textboxes on my form. The names are lbxSubjects, lbxCompanies, txtSubject and txtCompany repectively. I have drag and drop coded for each. The problem is when drag a item from the Subjects listbox not only can you drop it in the Subject textbox, it also will drop in the Company textbox. The same thing happens with Company also.

I want Subject to go Subject and Company to go Company. What am I doing wrong? This is my first drag and drop operation. Here is the drag and drop code.

private void txtSubject_DragDrop(object sender, DragEventArgs e)
{
    e.Effect = DragDropEffects.All;
}

private void txtSubject_DragEnter(object sender, DragEventArgs e)
{
    txtSubject.Text = (string)e.Data.GetData(DataFormats.Text);
}

  private void lbxSubjects_MouseDown(object sender, MouseEventArgs e)
{
    lbxSubjects.DoDragDrop(lbxSubjects.SelectedItem.ToString(),
    DragDropEffects.Copy);
}

private void txtCompany_DragDrop(object sender, DragEventArgs e)
{
    e.Effect = DragDropEffects.All;
}

private void txtFindCompany_DragEnter(object sender, DragEventArgs e)
{
    txtCompany.Text = (string)e.Data.GetData(DataFormats.Text);
}

private void lbxCompanies_MouseDown(object sender, MouseEventArgs e)
{
    lbxCompanies.DoDragDrop(lbxCompanies.SelectedItem.ToString(),
    DragDropEffects.Copy);
}

What I have tried:

I have read several articles on drag and drop but have nothing on this issue.