영어 문장 해석과 that, appearance to one 질문

두 문장 해석이 막혀서 질문드려요.
“Plenty of research has been done that shows how optimism plays a big role in higher achievement.”에서 done 뒤의 that은 어떤 역할인가요?
“Starting salaries ranged from 8 to 20 percent higher as the result of upgrading a mediocre business appearance to one that is polished and effective.”도 문장 구조와 해석이 궁금해요.
appearance to one이 어떻게 연결되는지 모르겠어요.