String and StringBuilder in C#.NET: select the correct statements. 1) Two strings can be concatenated using s3 = s1 + s2; 2) String is a primitive in C#.NET. 3) A string built with StringBuilder is mutable. 4) A string built with String is immutable. 5) Two strings can be concatenated using s3 = s1 & s2;

Introduction / Context:
This question reviews fundamental properties of string and StringBuilder and the standard concatenation syntax in C#.

Given Data / Assumptions:

• Statements to validate include concatenation forms and mutability claims.

Concept / Approach:
In C#, strings are immutable, and the + operator concatenates strings. StringBuilder provides a mutable buffer for efficient concatenations or edits. The & concatenation operator is a VB feature, not C#.

Step-by-Step Solution:

Verification / Alternative check:
Creating a StringBuilder and appending confirms mutability; attempting to modify an existing string shows it produces a new instance.

Why Other Options Are Wrong:
Options including 2 or 5 assert incorrect claims about primitives or use non-C# operators.

Common Pitfalls:
Assuming VB operators apply in C#; forgetting the performance implications of repeated + in loops.

Final Answer:
1, 3, 4