单选题设f(x，y)为连续函数，<img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14287738_0.png"><br/> <img border="0" alt="" src="https://tuku.weimei168.com/uploads/upimg/20240620/14287738_0_1.png">