单选题 Contrast may make something appear more beautiful than it is when ______ alone.
A. seen B. is seen C. to be seen D. having been seen

【正确答案】 A

【答案解析】[解析] 对比要比单独看某物更漂亮。
[解析] 考查分词的用法。在than引起的从句中，it和see之间是被动关系，因此要用过去分词seen；而且，分词可以直接跟在while或when的后面作状语，例如：Look out for cars when crossing the street．(过街时注意车辆。)