radio bottoms inside a group Box will be treated as a group of bottoms. They are mutual exclusive. How can I clean up their check states??

I have several radio bottoms, one of them are checked. How can I "clean" (uncheck) all radio bottoms?? "setChecked" doesn't work within a group, I tried to do following things but failed.

My code is as following, radioButtom is inside a groupBox, and I want to unchecked it. The first setChecked does works, but the second one doesn't, the radioBottom doesn't been unchecked

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QRadioButton *radioButton;
    ui->setupUi(this);
    radioButton->setChecked(true);
    radioButton->setChecked(false);
}

Where is the problem in my code?