Refer to the input text as context and select the correct relationship between 'AmEx' and 'American Express' from the available options.
Options: owner of, owned by, operator, employer

Sam’s Clubs to Accept AmEx Cards Earlier this year, retailer Costco cut ties with American Express after 16-year relationship By lisa.beilfuss@wsj.com Updated Sept. 10, 2015 5:41 p.m.
operator