None.
Yes. It crosses both of them, at 0 degrees longitude.
Amazonia
chad
danube
It actually pass through five african countries namely Namibia, South Africa, Zimbabwe, Botswana & Mozambique.
Circle
a circle
it has 41 countries
One
It means that if you take a circle and find its area, you must now find a square with the same area. We cannot square the circle.
australia
Infinite angles. If you are referring to degrees then the answer is 360, but if the question is angles I can get as many angles as I want inside a circle.