(validate) 3 checkbox

(validate) 3 checkbox

Hello

 I am begginer in jquery and I want to ask you
I have this form
http://www.wst.gr/form/index_en.html
based in this sample
http://jquery.bassistance.de/validate/demo/milk/  

As you can see I have 3 checkbox. How do I validate the user can choose at least 1 checkbox ?





 

Thank you

Sorry for my bad English