Which code fragment, inserted at line 24, outputs "123abc 123abc"?()
A.sb1.append("abc"); s1.append("abc");
B.sb1.append("abc"); s1.concat("abc");
C.sb1.concat("abc"); s1.append("abc");
D.sb1.concat("abc"); s1.concat("abc");
E.sb1.append("abc"); s1=s1.concat("abc");
F.sb1.concat("abc"); s1=s1.concat("abc");