I just need to configure reCaptcha, so it is only display numbers as display text,
no word as display text.
I need do this configuration in Nop
Google uses "advanced risk analysis techniques, actively considering the user’s entire engagement with the CAPTCHA" to determine how difficult of a puzzle the user deserves. So there is not configuration setting and most users get simple numbers by default.
Source: http://googleonlinesecurity.blogspot.com/2013/10/recaptcha-just-got-easier-but-only-if.html
I've noticed when testing for a while it starts to get suspicious of me and throws some that are near impossible to get right. Then a few days later it's nicer to me again.
Check this link Customize Google Re Captcha in Perl
No, this is not possible.
The reCaptcha system is meant as an easy way to plug in a Captcha to your site. As a side effect, it helps digitizing books by using image snippets from scans as a visual challenge. If only numbers should be displayed, then only such “words” from books could be used that are already known to be numbers. This would void the secondary objective of reCaptcha (helping the transcription of scans).
Using numbers only makes your system less secure (in theory). Whereas an attacking OCR software might have difficulty telling apart a g and 9, this now becomes unambiguous.