문제:괄호안에 들어갈 알맞은 접속사를 골라 동그라미하세요.

he needed the book not because he had to do homework with it,(and,but,or) because he had to return it.

여기서 정답은 but인데 그 이유를 잘모르겠습니다..좋은 답변 부탁드립니다 ㅎㅎ;