I have finished translating a large part of the book, but the rest more difficult.A. isB.

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(答案→)A. is
B. are
C. was
D. were
解析：
这题考查主谓一致，the rest做为主语，谓语要看the rest修饰的名词，这里the rest后面省略的是the book，所以谓语用单数，而且是一般现在时。如果the rest后面修饰的名词是复数，谓语动词是复数。