问题：

[单选]

import java.io.*； 
public class Forest implements Serializable { 
private Tree tree = new Tree（）； 
public static void main（String [] args） { 
Forest f= new Forest（）； 
try { 
FileOutputStream fs = new FileOutputStream（”Forest.ser”）； 
ObjectOutputStream os = new ObjectOutputStream（fs）； 
os.writeObject（f）； os.close（）； 
} catch （Exception ex） { ex.printStackTrace（）； } 
} 
} 
class Tree { } 
What is the result？（） 

A .  Compilation fails.
B .  An exception is thrown at runtime.
C .  An instance of Forest is serialized.
D .  A instance of Forest and an instance of Tree are both serialized.