Hide certain value in dropdown B when user choose certain value in dropdown A

I have 2 dropdown with same value.
​
I want value in dropdown B will hide when user choose the value in dropdown A.
​
Example:
user choose value 1, and same value in dropdown B will hide.
​
Dropdown A
value 1
value 2
value 3
​
Dropdown B
value 1
value 2
value 3
​
​
How can achieve this since I'm zero knowledge in programming.

Easiest way is to reload the second dropdown and use some code to determine which values you will display. Have a look at the "Dynamic dropdown" demo form.

Hi Healyhatman,
​
Both dropdown must be show on load.
But I only want to hide the certain value inside the the dropdown.
​
I have tested the Dynamic dropdown on demo form,
​
but the value in second dropdown not appear when the value is "?"
​

$options = [];
switch($this->data('what_do_you_think')){
 case '?':
 $options = ['2' => '2', '3' => '3', '4' => '4'];
 break;
case 'good':
 $options = ['2' => '2', '3' => '3', '4' => '4'];
 break;
 case 'bad':
 $options = ['1' => '1', '3' => '3', '4' => '4'];
 break;
 case 'so':
 $options = ['1' => '1', '2' => '2','4' => '4'];
 break;
       case 'test':
 $options = ['1' => '1', '2' => '2','3' => '3', '4' => '4'];
 break;
}
return $options;