1. 다음 글의 밑줄 친 부분 중, 어법상 틀린 것은?

the highest lake with islands is orba co in tibet. this lake has a surface ①that is 5,209 meters above sea level. tibet is also known as the “roof of the world.” it is a rough, dry, cold plateau north of the himalayas. it is ②bitterly cold in the winter and windy all year round. rain and melted snow flow into dozens of lakes, four of ③them are considered sacred by local people. there is very little vegetation in the stark, rocky landscape. the only trees are in the most sheltered valleys, and even ④those are often small. yet for all its barrenness, this land can be kind. humans live near the lakes, ⑤where they grow healthy crops.

정답은 3번 이라고 나오고 해설에 four of 다음에 목적격 대명사 them이 아니라 대명사와 접속사의 역할을 하는 관계대명사 which가 필요하다 라고 설명 되어있는데 이해가 잘 안됩니다 쉽게 설명 부탁드립니다