No oral surgeons are orthodontists, and all members of the Patterson Group are orthodontists. Ttherefore, no members of the Patterson Group are employees of the Davidson Consortium. The conclusion above is properly drawn if which one of the following is assumed? ['All employees of the Davidson Consortium are oral surgeons.', 'All orthodontists are members of the Patterson Group.', 'No oral surgeons are members of the Patterson Group.', 'No employees of the Davidson Consortium are oral surgeons.']