①用天平测出空玻璃杯的质量m₀；

②给玻璃杯中装满石英粉，测出玻璃杯和石英粉的总质量m₁；

③将石英粉倒出，给玻璃杯中装满水，测出玻璃杯和水的总质量m₂

④用已知量和测量量对应的字母写出石英粉密度的表达式．

其中④要求写出石英粉密度的表达式，这个表达式下列给出正确的是（   ）

   Mr. and Mrs. Clark are 70 years old. They look young and they are very healthy. Why? Let me tell you. That’s because they have a healthy lifestyle.
   They exercise every day, and every morning they exercise in the park. After that they usually go to the supermarket. They walk there for about half an hour. They don’t take buses. Because they think walking is good for their health. They often take a walk after dinner.
They also have a good eating habit. They drink milk every morning. They eat fruit and vegetables every day. As for meat, they only eat it twice a week. Besides (除…之外) that, they never eat junk food. They don’t drink coffee or cola at all. You see, they look after their health very well.
   Mrs. Clark likes watching soap operas and reading books. She reads books every day. Mr. Clark likes Chinese paintings. They usually go to bed at 9:00 pm and sleep for eight hours every night.

画出物体AB在平面镜中所成的像。