What is the relationship between Dick Costolo and Twitter in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

But Dorsey, who also founded Twitter in 2006, returned to the role of Twitter CEO in June this year after Dick Costolo said he planned to step down.

Answer: employer