LINQ Difference Between Questions List-2

1. Difference between First() and Single() extension methods
2. Difference between FirstorDefault() and SingleorDefault() extension methods
3. Difference between Count() and LongCount() extension methods
4. Difference between Xelement and Xdocument
5. Difference between OrderBy() and Sort() method over IList
6. Difference between Skip() and SkipWhile() extension methods
7. Difference between findall() and Where()
8. Difference between While() and TakeWhile() extension methods
9. Difference between SkipWhile() and Where() in LINQ
10. Difference between Enumerable.Select() and Enumerable.TakeWhile()
11. Difference between Concat() and Union() methods
12. Difference between LINQ to SQL and Entity Data Model(EDM)
13. Difference between Any and Exists in LINQ