quizze with radiobutton (4 questions) -- don´t work correctly

Hi.

I create a quizze with radiobutton but

If the user clicks the option rb8. label in question 4 and click any option for questions 1,2,3 ... always emerges CORRECT.

ie, only the radioGroup4 works.

stop();

import fl.controls.RadioButtonGroup;

var radioGroup1:RadioButtonGroup = new RadioButtonGroup("Question1");

var radioGroup2:RadioButtonGroup = new RadioButtonGroup("Question2");

var radioGroup3:RadioButtonGroup = new RadioButtonGroup("Question3");

var radioGroup4:RadioButtonGroup = new RadioButtonGroup("Question4");

rb1.group=radioGroup1;

rb2.group=radioGroup1;

rb3.group=radioGroup2;

rb4.group=radioGroup2;

rb5.group=radioGroup3;

rb6.group=radioGroup3;

rb7.group=radioGroup4;

rb8.group=radioGroup4;

rb1.label="";

rb2.label="a";

rb3.label="b";

rb4.label="";

rb5.label="";

rb6.label="c";

rb7.label="";

rb8.label="d";

submit_btn.addEventListener(MouseEvent.CLICK, submitClick);

function submitClick(event:MouseEvent):void{

  if((radioGroup1.selection==null)||(radioGroup2.selection==null)||(radioGroup3.selection==null)||(radioGroup4.selection==null)){

  return;

  }

  if(radioGroup1.selection.label=="a", radioGroup2.selection.label=="b", radioGroup3.selection.label=="c", radioGroup4.selection.label=="d"){

    status_txt.text="Correto... Parabéns.";

    }else{

    status_txt.text="Incorreto... Tente novamente."

}