Puzzle 1: Passing reference types by value (Beginner)

With this post I’ll start a series about programming puzzles and kata’s.
These small exercises will focus on the specific features of C#1,2 & 3. 
Have fun… 
 
What is the output of: ValueRef.Problem() ? 
Can you explain why?
 
  1: public class ValueRef

  2: {

  3:         public static void Problem()

  4:         {

  5:             StringBuilder myStrBuilder = new StringBuilder("x");

  6:             AppendHello(myStrBuilder);

  7:             Console.WriteLine(myStrBuilder.ToString());

  8:             Console.ReadLine();

  9:         }

 10: 

 11:         static void AppendHello(StringBuilder builder)

 12:         {

 13:             builder.Append("hello");

 14:             builder = null;

 15:         }

 16: }