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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"); 

(AB) sb1.concat("abc"); s1 = s1.concat("abc"); 

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

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