Why this error? C#, class inaccessible due to protection level

Question

https://code.sololearn.com/cM9S00A0580C/?ref=app

Answer 1

You have a static method 'hello()' within a public class 'hellow' wrapped in a namespace 'prog'. You can access it as:

hellow.hello();

or

prog.hellow.hello();

if you do not declare the use of namespace prog.

Answer 2

The method 'hello()' is in a different class (the hellow class), and therefore must be accessed as such. Because it is a static method, it can be accessed statically:
hellow.hello();

Answer 3

Put the class name (hellow) before its method (hello)

Answer 4

Hatsy Rei
it means, System is not a class?