Question

Hello, I have a problem that says 'graph the level curve of f(x,y) = sqrt(4 - x^2 - y^2) at c = 0, 1, and 2.'

I graphed these and checked my work with the answer in the back of the book. For c=0, I had a circle centered at the origin with radius 2, which I thought would be the base of the hemisphere. However, the book indicated there would not be a level curve at c=0 at all. I don't understand why this is.

Any help is appreciated. Thank you!