宁波杭州湾世纪城论坛:一道英语词汇题

来源:百度文库 编辑:查人人中国名人网 时间:2024/05/01 14:13:03
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
答案选A
请详细解释一下每个选项 说明其中的语法和时态

是一个句子分句中when后不用有谓语动词.