以下四个命题中：

①从匀速传递的产品流水线上，质检员每10分钟从中抽取一件产品进行某项指标检测，这样的抽样是分层抽样；

②两个随机变量的线性相关性越强，相关系数的绝对值越接近于1；

③若数据x₁，x₂，x₃，…，x_n的方差为1，则2x₁，2x₂，2x₃，…，2x_n的方差为2；

④对分类变量X与Y的随机变量k²的观测值k来说，k越小，判断“X与Y有关系”的把握程度越大．

其中真命题的个数为（　　）

“在智慧提供给整个人生的一切幸福之中，以获得友谊为最重要。”下列有助于建立友谊的方法有（    ）

①敞开心扉、主动表达    ②面带微笑、真诚夸赞

③尊重理解、关心支持    ④以诚相待，毫不保留

下列图形中，是中心对称图形又是轴对称图形的是（　　）

      Mr Green is an Englishman. He teaches English well His English classes are very interesting. He likes us and we like him, too.
Mr Green has two children----Mike and Nancy. Mike is eight and his sister Nancy is five. Mike goes to school but Nancy doesn't. Mr Green likes to wear a T-shirt and a pair of jeans. Football is his favorite sport. After school we often have a basketball match. Sometimes Mr Green watches and joins us. He plays basketball just for us.