Dynamic dropdown list

Hi experts,

Kindly solve my requirement.

There are 3 radio buttons R1, R2, R3.

i have a dropdown box with 2 values for a parameter in the selection screen.

When i select the first value , the radio button R1 & R2 should display in the selection screen and

when i select the second value ,the radio button R3 should display in the selection screen.

Thanks in advance.