[单选题]Given: Which code fragment, inserted at line 24,

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");

G sb1.append("abc"); s1 = s1 + s1.concat("abc");

H sb1.concat("abc"); s1 = s1 + s1.concat("abc");