单选题：当表示两个二元函数的线性组合时，下列哪种公式写法最符合规范？
当表示两个二元函数的线性组合时，下列哪种公式写法最符合规范？@[A](1)
A. $$c_{1}f_{1}(x_1,x_2)+c_{2}f_{2}(x_1,x_2)$$
B. $$af_{1}(x,y)+bf_{2}(x,y)$$
C. $$a_{1}f(x,y)+a_{2}g(x,y)$$
D. $$x_{1}f_{1}(a_1,a_2)+x_{2}f_{2}(a_1,a_2)$$
A.$$c_{1}f_{1}(x_1,x_2)+c_{2}f_{2}(x_1,x_2)$$
B.$$af_{1}(x,y)+bf_{2}(x,y)$$
C.$$a_{1}f(x,y)+a_{2}g(x,y)$$
D.$$x_{1}f_{1}(a_1,a_2)+x_{2}f_{2}(a_1,a_2)$$
答案：A
A. $$c_{1}f_{1}(x_1,x_2)+c_{2}f_{2}(x_1,x_2)$$
B. $$af_{1}(x,y)+bf_{2}(x,y)$$
C. $$a_{1}f(x,y)+a_{2}g(x,y)$$
D. $$x_{1}f_{1}(a_1,a_2)+x_{2}f_{2}(a_1,a_2)$$
A.$$c_{1}f_{1}(x_1,x_2)+c_{2}f_{2}(x_1,x_2)$$
B.$$af_{1}(x,y)+bf_{2}(x,y)$$
C.$$a_{1}f(x,y)+a_{2}g(x,y)$$
D.$$x_{1}f_{1}(a_1,a_2)+x_{2}f_{2}(a_1,a_2)$$
答案：A