Button的标题文字添加下划线

NSMutableAttributedString *c = [[NSMutableAttributedString alloc] initWithString:@"忘记密码?"];
    NSRange contentRange = {0, [c length]};
    [c addAttribute:NSUnderlineStyleAttributeName value:[NSNumber numberWithInteger:NSUnderlineStyleSingle] range:contentRange];
    //UIFontTools * fonttools = [UIFontTools defaultUIFontTool];
    //[self.btn_wangjimima.titleLabel setFont:[fonttools forgetPWDButtonFont]];
6   self.btn_skipVerification.titleLabel.attributedText = c;