optional checkbox automatically checked under certain condition

I am not using templates (yet), and I am a bit new at this.  I am using checkboxes and radio buttons.  CheckBoxA is optional.  Radio groupB (two buttons) is is optional.  However, if B2 is selected, CheckBoxA is required; I would like the selection of B2 to trigger the automatic selection of A.  Is this possible?  If not, is there some other way to set up this logic?  The condition options of A (show/hide/enable/disable) don't seem to accomplish this, and there seems no way through the settings on the B2.  THANK YOU for any help.