selection Screen

Hi ,

I want to Display Four Radio Buttons on the Selection Screen. But the Condition is that first it should show only two radio buttons and by selecting one of the it should show anohter two. I wrote it but it is not working . I wrote it in the At Selection-Output Event that if Screen-group = 'gr' the screen-invisible = 1.

After Execution of the program it shows only two buttons.But after selecting it not showing the remaiaing two buttons.

In which Event should I write the code ?

All these radio buttons are defined in the program.