一个关于TextWriter的问题。
这是MCTS   70-536的一个考题，

You   need   to   write   text   to   a   file.   Which   of   the   following   demonstrates   the   most   efficient   way   to   use   the   TextWriter   class?

A：
//   C#
TextWriter   tw   =   new   TextWriter( "Hello.dat ");
tw.Write( "Hello,   world! ");
tw.Close();

B：
//   C#
FileStream   fs   =   new   FileStream( "Hello.dat ",   FileMode.Create);
TextWriter   tw   =   new   StreamWriter(fs);
tw.Write( "Hello,   world! ");
tw.Close();
fs.Close();

C：
//   C#
TextWriter   tw   =   new   StreamWriter( "Hello.dat ");
tw.Write( "Hello,   world! ");
tw.Close();

D：
//   C#
TextWriter   tw   =   new   FileStream( "Hello.dat ",   FileMode.Create);
tw.Write( "Hello,   world! ");
tw.Close();

答案是B，解释是
The   TextWriter   class   does   not   have   a   constructor.   Instead,   you   should   create   it   using   the   StreamWriter   constructor.   To   create   a   StreamWriter   object,   you   must   use   an   existing   Stream   object,   such   as   an   instance   of   FileStream.

问题是StreamWriter是可以用文件名来直接构建对象的，C为什么不对呢？？我测试也没有问题的。

------解决方案--------------------
StreamWriter是TextWriter的派生类

估计是因为必须的用TextWriter的实例