单项选择题

程序: 
class TestReference{  public static void main(String[] args){  
int x=2; 
TestReference tr = new TestReference();         
System.out.print(x);         
tr.change(x); 
System.out.print(x); 
} 
public void change(int num){     
num = num + 1; } } 
程序运行后的输出是哪项?()  

A. 23
B. 21
C. 22
D. 编译错误