Match the surfaces with the verbal description of the level curves by placing the letter of the verbal description to the left of the number of the surface.

 1. $z = xy$
 2. $z = \frac{1}{x-1}$
 3. $z = 2x + 3y$
 4. $z = \sqrt{(25 - x^2 - y^2)}$
 5. $z = 2x^2 + 3y^2$
 6. $z = \sqrt{(x^2 + y^2)}$
 7. $z = x^2 + y^2$

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