更改用于UI按钮ios失去边框的Imagebackground

Question

当我将选择设置为NO时，为什么第一个按钮的边框会消失？以下是foreach按钮的两个操作。

- (IBAction)onehourbtn:(id)sender
{
    [btnonehour setSelected:YES];
    [btntwohour setSelected:NO];
    [btnthreehour setSelected:NO];
}

和

(IBAction)twohourbtn:(id)sender
{
    [btnonehour setSelected:NO];
    [btntwohour setSelected:YES];
    [btnthreehour setSelected:NO];
}