In the following Java code, which code fragment should be inserted at line 3 so that the output will be: "123abc 123abc"?

1. StringBuilder sb1 = new StringBuilder("123");
2. String s1 = "123";
3.  // insert code here
4. System.out.println(sb1 + " " + s1);

A. sb1.append("abc"); s1.append("abc");

B. sb1.append("abc"); s1.concat("abc");

C. sb1.concat("abc"); s1.append("abc");

D. sb1.append("abc"); s1 = s1.concat("abc");

Answer: Option D