A technical suggestion below, but first, I'd suggest that your real solution is to not use groups for the second question. Groups are set up nicely to handle mailing lists, but if it's to track interests, you'd be better off setting those up as custom fields. It'll solve this immediate issue, and it'll make it easier to deal with tandem searches and so forth (on list b and likes food d).
Now if you must have them as groups, you can create a fake field and move checkboxes into it using jQuery. Create the fake field with one option that you don't care about, but label it "What food are you interested in", or equivalent. Then, edit the Groups field that CiviCRM generated: label it more specifically as "which mailing lists...", and choose Static Options so it doesn't start offering up just any group for someone to choose.
Now, add the following javascript:
// first remove the dummy checkboxes in your fake field
$('#yourdummyfield .form-item').each( function() { $(this).remove(); });
// now move things into the dummy field
$('#yourdummyfield').append( $('#groupsfield .form-item-d');
$('#yourdummyfield').append( $('#groupsfield .form-item-e');
$('#yourdummyfield').append( $('#groupsfield .form-item-f');
From the form processing perspective, they'll all be evaluated as the "groups" field. However, they'll look separate. For better or worse, this will have to be adjusted as you add new groups fields.