B and F are correct. By marking the methods as synchronized, the threads will get the lock ofthe this object before proceeding. Only one thread will be setting or reading at any givenmoment, thereby assuring that read() always returns the addition of a valid pair. A is incorrect because it is not synchronized; therefore, there is no guarantee that thevalues added by the read() method belong to the same pair. C and D are incorrect; only objectscan be used to synchronize on. E fails— it is not possible to select other objects (even this) tosynchronize on when declaring a method as synchronized.