Hehe, I know you already solved your problem but just in case you decide that you feel like getting really complex, you could do this:

Set up a random phrase file format with various responses set up in a format something like this:

{"Congratulations!","Good for you!","Wow!","Amazing!","Excellent work!"}"You"{"got it"{"right!","correct!"},"answered the"{"problem","question"}"correctly."}

And something similar for a wrong answer.

Then, parse through the file and generate a random response from the different synonymous elements that can be fit together to make a valid English phrase.

I tried this once and it worked pretty well, though it took a long time to program and gave me plenty of headaches. With a large enough set of nested possible responses, the results can be quite random.