form_dropdown的工作方式与form_muliselect类似

I am using form_dropdown helper for creating select options dropdown. But after using set_select, it is working like form_multiselect.

Here is my code,

<?= 
   form_open();
   form_label('Approximate Day To Teach', 'Approximate Day To Teach'); 
?>
<?php  $approxDayOptions = [

        'Select'=>'Select',
        '1 Day'=> '1 Day',
        '2 Days'=> '2 Days',
        '3 Days'=> '3 Days',
        '4 Days'=> '4 Days',
        '5 Days'=> '5 Days',
        '6 Days'=> '6 Days',
        '7 Days'=> '7 Days',
    ];

    $approxDaySetCond = array(
        set_select('approxDayToTeach', '1 Day', TRUE),
        set_select('approxDayToTeach', '2 Days'),
        set_select('approxDayToTeach', '3 Days'),
        set_select('approxDayToTeach', '4 Days'),
        set_select('approxDayToTeach', '5 Days'),
        set_select('approxDayToTeach', '6 Days'),
        set_select('approxDayToTeach', '7 Days'),
    );
?>
 <?=
   form_dropdown('approxDayToTeach', $approxDayOptions, $approxDaySetCond, ['class'=>'form-control']),
   form_error('approxDayToTeach'),
   form_close();
?>

when I use above sample code , it works like form_multiselect, Please give me some suggestion regarding my question.

Set Default selected option as :

form_dropdown('approxDayToTeach', $approxDayOptions, '1 Day', ['class'=>'form-control'])

Use one of the options from (1 Day, 2 Day) instead of $approxDaySetCond array to avoid dropdown behaving like form_multiselect.