Seeing the chemical to the sunlight, Mr. Lee warned the workers to be careful when dealin

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A. to expose
B. exposing
(答案→)C. exposed
D. being exposed
解析：考查过去分词做宾语补足语，see+宾语+宾语补足语，the chemical和expose是被动关系，用过去分词做宾语补足语，句意：看见化学物质暴露于阳光下，专家警告工人处理的时候要当心。选C。