Refer to the input text as context and select the correct relationship between 'Dick Costolo' and 'Twitter' from the available options.
Options: director/manager, legal form, employer, manufacturer

Twitter co-founder Dorsey, who took over last month from Dick Costolo in the midst of criticism over the company's strategy, warned on Tuesday that Twitter was unlikely to see sustained growth in monthly active users (MAU) until it can reach the mass market.
employer