도대체 이게 왜 정답인지 모르겠어요. ㅠㅠ

It usually took five weeks for Benjamin Franklin in Paris (to receive / that receive)
a letter sent from Philadelphia.

여기서 정답이 to receive 라고 나오는데

도대체 왜 저게 정답인지 감이 안잡힙니다;; 도와주세요. ㅠㅠ 

왜 that이 오지를 못하는거죠;;

that은 뜻이 이상하게 되는건가요 아니면 문법 문제인가요