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。