What is the relationship between Sergio Marchionne and Fiat in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

UAW President Dennis Williams is seeking ratification of the tentative agreement he reached with Fiat Chrysler Chief Executive Officer Sergio Marchionne so he can move on to Ford Motor Co. and General Motors Co. to seek similar, but more lucrative deals.

Answer: employer