Each of the following functions has a set on which it is continuous and that set has a boundary. Match the verbal description of this boundary with the function by putting the letter of the boundary to the left of the letter of the function.

 1. $f(x, y) = e^{\frac{1}{x-y}}$
 2. $f(x, y, z) = \frac{xyz}{x^2+y^2-z}$
 3. $f(x, y, z) = \frac{z}{1-x^2-y^2}$
 4. $f(x, y, z) = \frac{1}{x^2+y^2+z^2}$
 5. $f(x, y) = \frac{1}{4-x^2-y^2}$

A. one point
B. a circular cylinder
C. a straight line
D. a circular parabaloid
E. a circle