Refer to the input text as context and select the correct relationship between 'Syriza' and 'Alexis Tsipras' from the available options.
Options: chief executive officer, distributed by, chairperson, product/material produced

Already, Prime Minister Alexis Tsipras is struggling to contain a rebellion in his leftwing Syriza party that made his government dependent on votes from pro-European opposition parties to get the tough bailout terms approved in parliament.
chairperson