Going to Mount Huangshan reminds me of the popular Beatles’ song “The Long and Winding Road”. ___56___ is so breathtaking about the experience is the out-of-this-world scenes. The rolling sea of clouds you see once you are at the top will remind you how tiny we ___57___ (human) are.

The hot spring at the foot of the mountain is something you must try after the climb. It will ___58___ (undoubted) help you get refreshed! The amazing thing about the spring is that the colder the temperature gets, the ___59___ (hot) the spring! Strange, isn’t it? But that’s how nature is—always leaving us ___60___ (astonish).

What comes next is the endless series of steps. You can’t help wondering how hard it ___61___ (be) for the people then to put all those rocks into place. Though it is the only unnatural thing on your way up the mountain, still it highlights the whole adventure ___62___ offers a place where you can sit down to rest your ___63___ (ache) legs.

As the song goes, this long and winding road “will never disappear”, and it will always stick in the visitor’s memory. It sure does in ___64___ (I). While you’re in China, Mount Huangshan is ___65___ must to visit!

这是2021高考英语新全国卷1的语法填空题。请问文中的 then 如何分析它的语法。