Given this method in a class：  public String toString（） {  StringBuffer buffer = new StringBuffer（）；  buffer.append（‟‟）；  return buffer.toString（）；  }  Which is true？（） 

A.  This code is NOT thread-safe.

B.  The programmer can replace StringBuffer with StringBuilder with no other changes.

C.  This code will perform well and converting the code to use StringBuilder will not enhance the performance.

D.  This code will perform poorly. For better performance， the code should be rewritten： return “”；