Although it was really a difficult task, he decided to try his best to success.A. challen

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A. challenge
B. host
C. approve
(答案→)D. achieve
解析：句意，尽管这的确是一项很艰巨的任务，但是他决定尽他最大的努力取得成功。challenge意思是挑战；host意思是主办；approve意思是赞同；achieve意思是取得。根据句意分析，本句要表达的意思是取得成功，故选D。