A$\displaystyle \lim _{(x, y) \rightarrow(0,0)}[f(x, y)-f(0,0)]=0$ .
B$\displaystyle \lim _{x \rightarrow 0} \frac{f(x, 0)-f(0,0)}{x}=0$ ,且 $\displaystyle \lim _{y \rightarrow 0} \frac{f(0, y)-f(0,0)}{y}=0$ .
C$\displaystyle \lim _{(x, y) \rightarrow(0,0)} \frac{f(x, y)-f(0,0)}{\sqrt{x^{2}+y^{2}}}=0$ .
D$\displaystyle \lim _{x \rightarrow 0}\left[f_{x}^{\prime}(x, 0)-f_{x}^{\prime}(0,0)\right]=0$ ,且 $\lim _{y \rightarrow 0}\left[f_{y}^{\prime}(0, y)-f_{y}^{\prime}(