How to visible and hide a drop down list on radio button choice?

Hi everybody!

I have the following problem and I hope that someone can help me.

I have a three radio-button choice on my PDF-form (R1; R2; R3) and the third one should visible or hide back an drop-down list.

So the code I made is making my drop-down visible, but when I´m choosing R1 or R2 after my R3 choice its not making my drop-down invisible again.

Can anyone see my mistake and help me?

Thank you in advance and sorry for my bad English

if (event.target.value != "Off") {
// box is checked
this.getField("List").display = display.visible ;
} else {
// box is unchecked
this.getField("List").display = display.hidden ;
}