Как объединить список > в C #

I'm having a List>, and which contains

{  {"A" , "B" }, 
   {"C" , "D" }
}

I need to union all the innerlist into another list

So the resulting List will contain

     {"A","B","C","D"}

Now im using for loop to do this

Is there any way to do this Using LINQ or Lambda Expression.

Please help me to do this.

Thanks in advance.