如图所示，一个圆柱形容器中装有一定量的水，一个质量为780g的石块挂在弹簧测力计上并浸没水中，石块静止时弹簧测力计的示数为4.2N，容器的底面积为180cm²，g=10N/kg．求：

根据材料内容，选择正确答案。

       I live in a small town near Xing'an in Guilin. You can't see it on the map of China, because it is too small. The air here is fresh(清新的). There are not many tall buildings in our town. The tallest building is in our school. There are four hundred students and twenty-five teachers in our school. In the front of the school, there is a playground. I often play basketball with my classmates on it. There is a little garden behind our school. And we can grow(种植) beautiful flowers and plant trees there. Next to the garden, there is an orange orchard(果园). You can hear birds singing everywhere. There is a river not far from our school. In summer, we usually go swimming with our teachers in it. We study Chinese, English, Maths and other subjects at school. The teachers are very nice. We love our school.

钠盐种类较多，在实验室里，钠盐通常都摆放在同一药品柜中，小华所在的化学兴趣小组在整理药品柜时，发现某种试剂瓶外壁两边贴上了新旧两个标签，分别是“Na₂SO₄溶液”、“Na₂CO₃溶液”，小华认为其原因是：Na₂CO₃溶液的试剂瓶混入了Na₂SO₄溶液，并加贴了新标签．该兴趣小组对溶液的成分进行了探究．请你完成以下探究过程，并将①～⑤处的答案填入答题卡相应位置．