Utilize the input text as a context reference, choose the right relationship between Willis Group and insurance broker from the options. Options: 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.

On Tuesday, insurance broker Willis Group Holdings and professional services provider Towers Watson unveiled an $18bn (£11bn) mega-merger, the 31st deal above the $10bn mark this year, and the highest number on record.

Answer: industry