5. The museum acquired a dozen paintings from two private donors, who were an art history professor at the local university.

　　(A) who were an art history professor

　　(B) who was each an art history professor

　　(C) who were both art history professors

　　(D) both of whom were an art history professor

　　(E) which were both art history professors

　　答案：C

　　类型：名词单复数

　　解析：定语从句的先行词是two private donors，所以不能用an art history professor来指代。ABD可以直接排除。E选项which使用错误，which不能指人。

　　6. To attract prey, a fish known as the red grouper restructures the ocean floor, digging holes to create the type of environment favored by smaller fish and spiny lobsters.

　　(A) digging holes to create

　　(B) digging hole creates

　　(C) digging holes create

　　(D) it digs holes creating

　　(E) they dig holes to create

　　答案：A

　　类型：非谓语

　　解析：主句是a fish…the ocean floor，digging holes做状语，to create表目的。原句没有错误。B中creates使用错误，C中create使用错误，dig holes与create有目的关系。D和E是run-on句子，都错。