What is the relationship between 'Syriza' and 'Alexis Tsipras' in the context of the input sentence.
Options: distributed by, chairperson, product/material produced, chief executive officer

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