Element ui select cleanup option

how can select empty the selected data and display the original placeholder??
here is my code:

Front-end javascript vue.js element-ui
Mar.03,2021 
form.city='----'

this.form.city='';

this.form.city=null;

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1c20503-32d2e.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1c20503-32d2e.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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