The policemen had no in finding the knife that the murderer had thrown. A. difficultyB.

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(答案→)A. difficulty
B. troubles
C. problems
D. question
解析：句意：警察毫不费力地找到了杀人犯扔掉的刀子。have difficulty/trouble (in) doing sth. 做某事毫不费力，是一个固定短语，此短语要须注意，difficulty, trouble都是不可数名词。