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

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

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