What is the relationship between grocery and Walmart 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.

Walmart is also testing an online grocery service in five cities — San Jose, Calif., Denver, Bentonville, Ark., Phoenix and Huntsville, Ala. Customers can order groceries online, then schedule a pick-up time.

Answer: operator