Using MDI Form closing and opening of forms in C#

Question

Using MDI forms closing of all background forms, whenever a new form is opened.

1. MDI forms consists of multiple forms (Form-1,Form-2,.....,Form-n)
2. If i open three forms (Form-1, Form-4, Form-5)
3. Suddenly i need to work on Form-9 and I open on Form-9.
4. But at Background there are three forms opened that should be closed whenever a new form is opened (i.e, Form-9).

Using MDI Forms..

 

So if there's a solution or link please send me.....