对具有线性相关关系的变量x，y有一组观测数据(x_i，y_i)( i=1，2，…，8)，其回归直线方程是$\stackrel{\wedge }{y}=\frac{1}{3}x+a$：，且x₁+x₂+x₃+…+x₈=2(y₁+y₂+y₃+…+y₈)=6，则实数a的值是( )

    I am John. I always get up late at the weekend. At about 9:30 am, I have breakfast. After breakfast, I start doing my homework. We have lunch at about 12:00, and then I watch TV or have a rest. I like reading and I often read some interesting books in the afternoon. I like sports, too. Sometimes I play football or basketball with my friends at school. My school is near my home, so I can walk there. Every weekend, my father takes me to a restaurant for a big dinner. I can enjoy lots of nice food. I like my weekend.

世界大多数动植物都能在我国找到适合生长的地区，是因为我国（    ）

 为准确配制80g 10%的氯化钠溶液，要求尽量减小计量仪器称量时的误差．该实验用到了下列仪器，其中属于计量仪器的是（　　）