Given that the current directory is empty, and that the user has read and write permissions, and the following:

Which statement is true?（）

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");

A.2
B.3
C.4
D.6
E.7
F.Compilation fails.

A.Foo { public int bar() { return 1; }
B.new Foo { public int bar() { return 1; }
C.new Foo() { public int bar() { return 1; }
D.new class Foo { public int bar() { return 1; }