执行下面代码的输出结果是( )。 Boolean b1 = new Boolean(true); Boolean b2 = new Boolean(true); Object obj1 = (Object)b1; Object obj2 = (Object)b2; if (obj1 == obj2) if (obj1.equals(obj2)) System.out.println("a"); else System.out.println("b"); else if (obj1.equals(obj2)) System.out.println("c"); else System.out.println("d");