What version of WeBWorK is this for?
A radio button interface is only for one correct answer. It's like an old radio where you push one button down, and that automatically pops up any other button that was depressed.
What you want is checkboxes. Prior to 2.18, there was a tool for checkboxes in PGchoicemacros.pl, but it had certain issues. With 2.18, there is parserCheckboxList.pl.