Thank you.
Upps, helps a lot. It was so simple. I did not about the two possibilities for the answer parameter. Where I can find that kind of information? or may I check the MathObjects library?
WeBWorK Problems
RadioButtons fails if answer is 0
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day