單項選擇題

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; } } 
程序運行后的輸出是哪項?()  

A. 23
B. 21
C. 22
D. 編譯錯誤

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