I am an English girl. My name is Mary. I’m twelve. My dad and mom are English teachers. I have a brother（哥哥）. His name is Tom and he is thirteen years old. We are in China now. We are in Beijing No. 4 Junior High School. Tom is in Class 4, Grade 7 and I am in Class 9, Grade 7. I have a dog. My dog looks strong. It has a big mouth and two big black ears. We have many(许多) Chinese friends here. They are good.

成语大比拼。

坚定____      贫穷____

当晨昏圈与经线圈重合时，下列说法正确的是（　　）

一门反坦克炮瞄准一辆坦克，开炮后经过0.6s看到炮弹在坦克上爆炸，再经过2.1s听到爆炸的声音，

求：（1）大炮距坦克多远？

（2）炮弹的飞行速度多大？（忽略光传播手所用的时间）