The firm has just accepted a major commission from johnson and partners, and all staff will [ ] be asked to work some extra hours next week.

1. similarly
2. besides
3. because
4. therefore

여기에서 왜 답이 therefore인가요?? therefore이 답이려면 and가 사라지고 주어인 all staff가 therefore의 뒤에 나와야하니까

The firm has just accepted a major commission from johnson and partners, therefore all staff will be asked to work some extra hours next week.

이어야 하지 않나요??


궁금한 것은 두가지입니다.

1. Therefore이 답인 이유는???
2. And가 존재하는 이유는???