下列图示实验操作中，正确的是（　　）

在一个等差数列$\left\{a_n\right\}$中，$\frac{a_n}{a_{2n}}$是一个与n无关的常数，此常数的集合是 （   ）

    Hello! I'm Alex. I often get up at 6:30 am. Half an hour later(之后), I have my breakfast. At 7:30 a.m., I take a bus to school. We have Maths, Chinese and English classes from 8:00 a.m. to 11:30 a.m. I have lunch and talk with my classmates at school. We have three classes from 1:40 to 4:10 in the afternoon.

    After school, I often play with my classmates. We often play soccer, tennis and ping-gong. I go home at about 5:00 p.m. I usually have dinner at 6.00 p.m. Then I do my homework and watch TV. At about 10:30 p.m., I go to bed.

关于经线和纬线的正确叙述（　　）