which，although 还是 though

This green Great Wall, _____ at its peak during theQing Dynasty covered thousands of hectares, had by the 1950s virtually disappeared.

老师好，这一句话答案是填which，能否填although或者though吗？

不能。covered在这里是主动意义，是过去式作谓语，从句缺少主句，所以要用which作主语。though或者although为从属连词，不作成分，所以用了后从句缺少主语。让步从句虽然能省略，但省略的是主语+be, 不可以只省略主语。所以用though或although为语法错误。