c# .net coding for textbox and radio button

Hai, I am coding in C# using visual studio 2010.

In my application i want a link between a text box in one form and radio button in another form.

Please send the code for it.

I want text Box must hold the selected value of radiobuttons.

In my application there are 3 radio buttons in one form.And these three must select automatically based on the values of textbox in another form.

for example textbox is for students marks and radio buttons for grades.If in textbox marks are above 70 then first radio button will automatically select(as A grade).or if marks or above 60 then second radio button will automatically select(as B grade). something like this...