Which of the following statements are correct about the String Class in C#.NET?

1. Two strings can be concatenated by using an expression of the form s3 = s1 + s2;
2. String is a primitive in C#.NET.
3. A string built using StringBuilder Class is Mutable.
4. A string built using String Class is Immutable.
5. Two strings can be concatenated by using an expression of the form s3 = s1&s2;