经济文化比较落后的黔东南,要确保2020年与全国全省同步全面建成小康社会的宏伟目标,具有决定性意义的一条就是把经济建设转移到　　　　的轨道上来。(　　)

①依靠科技进步               ②提高劳动者素质

③开发利用自然资源         ④团结各族人民

    Hello, my name is Betty and I'm in Class Six. I'm twelve years old. I'm American. I'm from New York. Mike is in my class and he is my good friend.

    Hi, I'm Mike. I'm from London. I'm in Class Six, too. Wang Tao is my friend. He is in my class.

    Hi, my name is Wang Tao. I'm Chinese. I'm from Hangzhou. I'm in Class Six with Betty and Mike. They are my good friends.

（Ⅰ）当实数m取什么值时，复数z是纯虚数；

（Ⅱ）当m=0时，化简 $\frac{z^2}{z+5+2i}$ ．