问题：

[单选]

public class MyLogger { 
private StringBuilder logger = new StringBuuilder（）； 
public void log（String message， String user） { 
logger.append（message）； 
logger.append（user）； 
} 
} 
The programmer must guarantee that a single MyLogger object works properly for a multi-threaded system. How must this code be changed to be thread-safe？（） 

A .  synchronize the log method
B .  replace StringBuilder with StringBuffer
C .  No change is necessary， the current MyLogger code is already thread-safe.
D .  replace StringBuilder with just a String object and use the string concatenation （+=） within the log method