public class X implements Runnable { private int x； private int y； public static void main（String [] args） { X that = new X（）； （new Thread（ that ））.start（）； （new Thread（ that ））.start（）； } public void run（） { for （；；） { synchronized （this） { x++； y++； } System.out.println（Thread.currentThread（）.getName（） + “x = “ + x + “， y = “ + y）； } } } What is the result？（）

A. Compilation fails.
B. The program prints pairs of values for x and y that might not always be the same on the same line （for example， “x = 2， y = 1”）.
C. The program prints pairs of values for x and y that are always the same on the same line （for example， “x = 1， y = 1”）. In addition， each value appears only once （for example， “x = 1， y = 1” followed by “x = 2， y = 2”）. The thread name at the start of the line shows that both threads are executing concurrently.
D. The program prints pairs of values for x and y that are always the same on the same line （for example， “x = 1， y = 1”）. In addition， each value appears only once （for example， “x = 1， y = 1” followed by “x = 2， y = 2”）. The thread name at the start of the line shows that only a single thread is actually executing.