答案：(D)分析：which既引导从句又作从句主语，it作主语属多余，

答案：(D)分析：which既引导从句又作从句主语，it作主语属多余，故去掉。改正：which it → which参考译文： 到1860年，美国的铁路有了 3,000 英里的轨道，其中的四分之三位于密西西比河东部和俄亥俄河北部。

Kết quả (Anh) 1:

Answer: (d) analysis: which introduce a clause and clause subject, it made redundant, it is removed. Correction: which it → which text: in 1860, United States Railroad has 3,000 miles of track, of which three-fourths is located in the East of the Mississippi River and North of the Ohio River.

Kết quả (Anh) 2:

Answer: (D) analysis: which both introduce a clause for clause and the subject, it superfluous as subject, it is removed. Correction: which it → which reference translation: 1860, the United States had 3,000 miles of railroad track, where three-quarters located east of the Mississippi and Ohio River North.

Kết quả (Anh) 3:

(D): () analysis: which not only to guide the subordinate clause and the clause subject, the subject of the it is redundant, so remove. By 1860, the United States has 3000 miles of track, and the 3/4 is located in the east of the Mississippi River and the of Hebei,.