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.

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

The em dash (—) is a versatile punctuation mark, as it can replace several other marks. For example, the em dash can take the place of commas to enhance readability and to emphasize a particular clause. Em dashes can also be used in order to replace parentheses (when used at the end of a sentence, only one dash is necessary). Finally, an em dash can replace a colon.

a. beside the fence

explanation: