The method foo() is private, so you don’t inherit it and can’t call it directly from the C class.
However, you can see private methods and constructor from B since everything is declared in the same containing class, and access them with super, which is why super() works.
In the same way, you can access foo with super.foo().
Note that you can redefine a new foo method in C, but this method will not override B.foo().