The book by him is difficult .
A. wrote; understanding
B. written; to be understood
C. writing; understand
D. written; to understand
网友回答
A. wrote; understanding
B. written; to be understood
C. writing; understand
(答案→)D. written; to understand
解析：考查非谓语动词，第一空是过去分词作定语用written，difficult后用不定式主动表被动。句意：被他写的这本书很难理解。所以选D。