Utilize the input text as a context reference, choose the right relationship between 'Sheryl Sandberg' and 'chief operating officer' from the options.
Options: industry, position held, owner of, member of

MOBILE BOWL : Sheryl Sandberg , chief operating officer at Facebook, took to the Advertising Week stage on Tuesday to champion mobile advertising’s ability to reach a big audience with targeted ads, CMO Today reports.
position held