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为什么不选B，而选A，请回答详细一点
热心网友
如果when 后面有主语，例it，那就是选Ｂ，如果省略了主语，也要省略is
热心网友
简洁！如果是用B,应该：when it is seen alone.
热心网友
简洁！如果是用B,应该：when it is seen alone.