A、are difficult
B、have been found difficult
C、is supposed difficult
D、has proved difficult
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正确答案
试题解析
句意:这本书尽管第一部分很容易,但是剩余部分被证明很难。句中将书内容分成两部分,因此“剩余部分”是一个整体,谓语用单数;be supposed之后接不定式,即be supposed to do/be...意为“被认为应该…”,因此C项正确的表达为is supposed to be difficult。故答案是D项。