Which sentence below shows correct use of em dashes?

In a-thousand-years, most of our greatest monuments such as they are will have crumbled to nothing.
In a thousand years, most of our greatest monuments – such as they are – will have crumbled to nothing.
In a thousand years, most of our greatest – monuments such as – they are will have crumbled to nothing.
In a thousand years, most of our greatest monuments such as they are will have crumbled-to-nothing.

Question

Which sentence below shows correct use of em dashes?

 In a-thousand-years, most of our greatest monuments such as they are will have crumbled to nothing.
 In a thousand years, most of our greatest monuments – such as they are – will have crumbled to nothing.
 In a thousand years, most of our greatest – monuments such as – they are will have crumbled to nothing.
 In a thousand years, most of our greatest monuments such as they are will have crumbled-to-nothing.

anonymous · Answer

I believe it is a when I used the elimination key