Match the surfaces $z = f(x,y)$ with the verbal description of the level curves $(f(x,y) = k,\ k = 1, 2, 3, ...)$ by placing the letter of the verbal description to the left of the number of the surface.

 1. $z = \frac{1}{x-1}$
 2. $z = xy$
 3. $z = \sqrt{(x^2 + y^2)}$
 4. $z = 2x^2 + 3y^2$
 5. $z = x^2 + y^2$
 6. $z = 2x + 3y$

A. a collection of unequally spaced parallel lines
B. a collection of equally spaced concentric circles
C. a collection of hyperbolas
D. a collection of equally spaced parallel lines
E. a collection of unequally spaced concentric circles
F. a collection of concentric ellipses