The largest set on which the funtion $f(x,y) = \sqrt{x+y} - \sqrt{x-y}$ is continuous is

A. $\lbrace (x,y) | x \ge y \rbrace$
B. $\lbrace (x,y) | -x \le y \le x \rbrace$
C. the whole xy-plane
D. $\lbrace (x,y) | -x < y \le x \rbrace$
E. $\lbrace (x,y) | -x < y < x \rbrace$