城市化对自然地理环境的影响有(       )
①温室气体排放多，出现“热岛”现象       ②改变局部大气环流，使降水减少
③城市建筑物面积扩大，增加了生物多样性 ④城市建设使地面径流汇集速度加快

已知函数$\left\{\begin{array}{c}x+k\left(1-a^2\right),\left(x\ge 0\right)\\ x^2-4x+{\left(3-a\right)}^2,\left(x<0\right)\end{array}\right.$，其中a∈R．若对任意的非零实数x₁，存在唯一的非零实数x₂（x₁≠x₂），使得f（x₁）=f（x₂）成立，则k的取值范围为（　　）

根据材料内容，选择正确答案。

    Mr Smith had a week's holiday, so he said, "I'm going to see the sea by train." He put on his best clothes, took a small bag, went to the railway station and got onto the train. He wore a beautiful hat, and he often put his head out of the window during the trip and looked at the beautiful scenery. But suddenly the wind pulled his hat off.

    Mr Smith took his old bag and threw it out of the window quickly too. The others in the train laughed. "Is your bag going to bring back your beautiful hat?" they asked.

    "N0," Mr Smith answered, "but there is no name or address in my hat, and there's a name and an address on the bag. If someone finds both of them, he or she will send me the bag and the hat.''