答案是A。你先把此句改为肯定句,就知道此句主句缺少先行词This factory is(the factory)the factory 可以用the one 替代,省略了关系代词that.
我给你你想要的另外两个看起来很像的句子1.Is this the factory----you visited last year? 则用which/that2.Is this the factory----you worked last year? 则用where需要详细讲解再说