Check out example codes for "html select multiple". It will help you in understanding the concepts better.

Code Example 1

To select multiple options you have to use 'multiple' attribute in select dropdown

Selecting multiple options vary in different operating systems and browsers:
For windows: Hold down the control (ctrl) button to select multiple options
For Mac: Hold down the command button to select multiple options

Html:
<select name="direction" id="directions" multiple>
  <option value="east">East</option>
  <option value="west">West</option>
  <option value="north">North</option>
  <option value="south">South</option>
</select>

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