Utilize the input text as a context reference, choose the right relationship between 'Dick Costolo' and 'Twitter' from the options.
Options: owner of, brand, developer, employer

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