In the context of the input sentence, determine the relationship between 'Dick Costolo' and 'Twitter'.
Options: employer, brand, owner of, developer

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.
employer