Take context from the input text and decide on the accurate relationship between 'Syriza' and 'ATHENS' from the options provided.
Options: operator, product/material produced, headquarters location, legal form

ATHENS - Greek Prime Minister Alexis Tsipras, struggling to contain a revolt in his left-wing Syriza party, said on Wednesday that his government would not implement reform measures beyond those agreed with lenders at a euro zone summit this month.
headquarters location