The most popular such method is CAPTCHA.

最常见的一种方法是 CAPTCHA。

Figure 3 is an example of a CAPTCHA image.

图 3 是 CAPTCHA 图像的一个例子。

Are some CAPTCHA devices better than others?

是否有些CAPTCHA设备比其他设备更好？

I want users to have to fill in a captcha to comment.

我希望用户必须要回答captcha才能写注释。

The image is then presented as a CAPTCHA in the usual way.

然后这个图片以常规方式显示为验证码。