No, you're not. A captcha already knows the correct answers. Otherwise, it wouldn't know if your selections are correct.
Besides, Google doesn't need help in picking out pictures of motorcycles or buses or bridges.
Besides, Google doesn't need help in picking out pictures of motorcycles or buses or bridges.
I expect the idea is the same with images, if there are 4 buses I expect 2 it knows are correct and used as a sanity test, 1 it's pretty sure it's correct (from majority of users selecting it), and 1 has some heuristics that might be a bus but it is currently testing to see whether users believe is or not. Similarly with the 5 not a bus images.
It doesn't need to know that your selections are "correct", only that they match responses given by most other users.
My intent was to disabuse the notion that captchas are how Google trains for image recognition.