Clear all in a multi-select drop down

i'm designing a form that includes this multi select combo box. by default all the options in it are selected. but when filling the form the user will be required to select 1-2 options. this form would have around 30 options in it.

in order to do this the user will probably need to clear the form. i'm curious whats the best way to clear the options in the combo box once the user has opened it. maybe some kind of clear all button, but how would it be within the combo box