题目33 [单选题] 二元函数z=3(x+y)-x^3-y^3的极值点是( )A. (1,2)B. (1,-2D)C. (1,-1)D. (-1,-1)33 [单选题] 二元函数$z=3(x+y)-x^{3}-y^{3}$的极值点是( )A. (1,2)B. (1,-2D)C. (1,-1)D. (-1,-1)题目解答答案D. (-1,-1)