The tundra is a harsh environment. Only the toughest plants and animal can survive.
Joining words:
And.
Or,
But,
So

The tundra is a harsh envioroment so only the toughest plants and animals can survive.

The tundra is harsh environment so, Only the toughest plants and animals can survive.

Explanation

Just read them i guess and see which makes more since

the purpose of a nonfiction text is to give the text a rap up. for example, when i say : frogs are interesting animals is an ending. then when i say: frogs are unique in different ways. that is an intro.

b is the answer.   i sat on the lumpy sofa, totally exhausted, with a bad taste in my mouth, wondering what i should do next.