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Thanks. I install as instructed. It seems to be working but there are some problems:
If you keep num_chars as 4, reCaptcha looks for first 4 letters.
So I set num_chars as 24 instead of 4, but it does not solve problems.
If first word has N letters, and if you write only first N-1 letters of first word (and it does not care of second word) than you will passed by reCaptcha. But this is not what we want.