It used to be there were two variables one that was uncertain to the machine and the other that was known. The known variable was used to ensure you weren't making up your answer.
But for at least a couple of months now I haven't seen a two variable captcha. I can only assume every captcha has been solved and verified to a reasonable degree of certainty. If Google, who is probably most able to benefit off of captcha solutions is willing to move past it I can't really argue with them.
I think a lot of the single-challenge CAPTCHAs they're using now are house numbers on Street View. Even if Google doesn't know the exact answer, they can rule out a lot of wrong answers (e.g, the number entered is on the wrong side of the street, or is completely out of range for the current block).
Also, if you look really suspicious (particularly if you fail an easier CAPTCHA, or ask for lots of challenges), Google will still give you one of the old two-scrambled-words CAPTCHAs. Except both of the words are ones they know the answers for, and you'll have to enter them both correctly to pass.
But for at least a couple of months now I haven't seen a two variable captcha. I can only assume every captcha has been solved and verified to a reasonable degree of certainty. If Google, who is probably most able to benefit off of captcha solutions is willing to move past it I can't really argue with them.