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.
Text: 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.
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