此处the last time 一定要接完成时吗?
在文章中看到一句话：In a series of experients the scientists showed the volunteers logos from several fast-food chains or asked them the last time they'd visited.
我想问的是，最后几个词“last time they 'd visited”为啥用完成时而不用过去式？用the last time they visited 可否？此处依据是什么？
是否两者可以随意转换？