问题：

[单选]

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.