c# - Why am I getting an "inaccessible due to protection level" error? -

I am getting this error:

'CTest.AA ()' is unavailable

  Public class A {Private A () {}} Public class B: A {public Zero Sehelo () {console .WriteLine ("Hello"); }}   
 Why can anyone explain why?  
  
 
 
 By default the constructor for A is private, as a constructor  protected A () {}  Try it.  
 Class  B  automatically calls the default constructor of  A , if it is not accessible for  B  Or there is no default constructor (if you have a constructor  protected A (string s) {} )  B  can not be started correctly.  
 The compiler automatically generates the following default constructor in           
 Where is the real call to the constructor of  base ()  real  a .