The TEXT statement specifies text for the dialog box that appears on line 8 (#8) and begins in column 1 (@1). The RADIOBOX statement specifies that a radio box will appear in the dialog box. DEFAULT= specifies that the first radio button ( Pollutant A) will be selected by default. The RBUTTON statements specify the mutually exclusive choices for the radio buttons: Pollutant A or Pollutant B. SUBSTITUTE= gives the value that is substituted for the %2 in the preceding DIALOG statement if that radio button is selected. 
          text #8 @1 'Choose a contaminant:';
          radiobox default=1;
             rbutton #10 @5  'Pollutant A' substitute='pol_a,2';
             rbutton #11 @5  'Pollutant B' substitute='pol_b,4';