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.

Twitter's disappointing first-quarter results eventually led to Dick Costolo stepping down as CEO and Jack Dorsey coming in as interim CEO while the company continues to look for a permanent CEO.Since April, shares of Twitter have dropped more than 34% amid concerns about slowing user growth.

Answer: employer