For each function below, consider its level surfaces, where $w=\cdots -2,-1,0,1,2\cdots$ if negative $w$'s are allowed, or $w = 1,2,3,\cdots$ if negative $w$'s are not allowed. Match each function with the verbal description of its level surfaces by placing the letter of the verbal description to the left of the number of the function.

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

A. a collection of unequally spaced concentric spheres
B. a collection of equally spaced parallel planes
C. two cones and two collections of hyperboloids
D. a collection of unequally spaced parallel planes
E. a collection of equally spaced concentric spheres
F. a collection of concentric ellipsoids