宜春樟树“四特酒”素有“三杯下肚浑身爽，一滴沾唇满口香”的赞誉，它的包装盒上标明容量500mL．（ρ_酒=0.9×10³kg/m³），则它装满酒的质量为　 ____　kg；如果用此瓶装满水，则总质量比装满酒时多　 ____　g．

    In most parts of the world, many students help their schools make less pollution. They join “environment clubs”. In an environment club, people work together to make our environment clean.

    Here are some things students often do.

    No-garbage lunches. How much do you throw away after lunch? Environment clubs ask students to bring their lunches in bags that can be used again. Every week they will choose the classes that make the least garbage and report them to the whole school.

    No-car day. On a no-car day, nobody comes to school in a car. Not the students and not the teachers! Cars give pollution to our air, so remember: walk, jump, bike and run. Use your legs! It's lots of fun.

    Turn off the water! Did you know that some toilets can waste twenty to forty cubic (立方) meters of water an hour? In a year, it would fill a small river! In environment clubs, students mend those broken toilets. We love our environment. Let's work together to make it clean.

① $\frac{4}{7}$  ② $\frac{3}{7}$  ③ $\frac{9}{7}$  ④ $\frac{16}{7}$ ．