The week's deals included the $28.3 billion purchase of insurer Chubb by Swiss insurer Ace Limited (NYSE: ACE - news ) ; the $6.8 billion acquisition of Health Net (NYSE: HNT - news ) by rival health insurer Centene; and the decision by US professional services company Towers Watson and insurance broker Willis Group to combine in a deal with an implied equity value of about $18 billion.
Analyze the relationship between 'Willis Group' and 'insurance broker' within the context of the input sentence.
Options: chief executive officer, headquarters location, industry, stock exchange