如图所示，两个皮带轮的转轴分别是O₁和O₂，设转动时皮带不打滑，则皮带轮上的A、B、C三点运动快慢关系是（   ）

完形填空

    The lion is one of the most fierce (凶猛的) animals in the world. Lions can be 1 in Africa and India. Lions in Africa can live without water for about one month. Lions are also popular animals in the 2 . Almost every zoo around the world has a few lions.

    It is very 3 for people to tell male and female lions from each other. You can judge (判断) simply by their appearance. A male lion has mane while female lions do not have. Male lion 4 than female lions. A male lion usually weighs about 200 kilograms which is always heavier than a female lion. Both male and female lions have very 5 mouths. They can break the backs of other animals 6. Lions feed on other animals and one lion usually kills between ten and twenty large animals each year for 7 .

    It's unusual for lions to kill people. However, in 1898 two lions 8 and ate over one hundred people in Kenya before they were shot by a British man named P.J. Patterson. This 9 became the subject (主题) of a 1996 Hollywood movie called The Ghost and the Darkness. Now the Indian lions are endangered. It's really necessary for us to 10 the animals, or we won't see them again. Animals are our friends. It is our obligation to protect them.

在下列命题中，正确命题的个数为（　　）
①两个复数不能比较大小；
②$z_1,z_2,z_3\in C$，若${\left(z_1-z_2\right)}^2+{\left(z_2-z_3\right)}^2=0$，则z₁=z₃；
③若(x²-1)+(x²+3x+2)i是纯虚数，则实数$x=\pm 1$；
④z是虚数的一个充要条件是$z+\stackrel{-}{z}\in R$；
⑤若a,b是两个相等的实数，则(a+b)+(a+b)i是纯虚数；
⑥$z\in R$的一个充要条件是$z=\stackrel{-}{z}$．