Please explain why s1 == s2 is false and why s2 == s4 is true. (drawing the picture of heap and stack gonna be more helpful ! )

Assume the following code segment:

String s1 = "hello"; String s2 = s1; String s3 = new String( "hello" ); String s4 = "hello"; String s5 == new String( "hello" ); s1 = "hello world";

Which of the following statements is true?

a) s1 == s2 is true b) s3 == s4 is true c) s3 == s5 is true d) s2 == s4 is true