A student wants to verify the upper fixed point and the lower fixed point on a thermometer with a Celsius scale. She has four beakers $P$, $Q$, $R$ and $S$. Beaker $P$ holds a mixture of ice and salt. Beaker $Q$ holds a mixture of ice and water. Beaker $R$ holds boiling salt solution. Beaker $S$ holds boiling water. Which two beakers should she choose to check the fixed points?
- A$P$ and $R$
- B$P$ and $S$
- C$Q$ and $R$
- D$Q$ and $S$