다음 글의 밑줄 친 부분 중,어법상 틀린 것은?

she found that while her iq is above average, her numerical ability is similar to that of an 11-year-old
because she has a disease called dyscalculia. she was partly relieved to know the truth because of it explained a
lot of difficulties she had in her daily life.

답은 5번 because 가 틀린부분 그러니까 정답이 되겠네요
왜 틀렸는지 설명 부탁드리겠습니다.~