下列极限存在的是 （ ）A. lim_((x,y) to (0,0)) (x)/(x+y)B. lim_((x,y) to (0,0)) (1)/(x+y)C. lim_((x,y) to (0,0)) (x^2)/(x+y)D. lim_((x,y) to (0,0)) x sin (1)/(x+y)

A. $\lim_{(x,y) \to (0,0)} \frac{x}{x+y}$

B. $\lim_{(x,y) \to (0,0)} \frac{1}{x+y}$

C. $\lim_{(x,y) \to (0,0)} \frac{x^2}{x+y}$

D. $\lim_{(x,y) \to (0,0)} x \sin \frac{1}{x+y}$