A tube shaped like a W contains two separate portions of mercury, and each is exposed to the atmosphere. A pocket of air at pressure $P$ is trapped between them. The diagram labels two vertical distances $x$ and $y$. Atmospheric pressure is the same as the pressure produced by a column of mercury $760\,\text{mm}$ high. The pressure $P$ is written in this form. Which combinations of $x$, $y$ and $P$ could be correct?
- A$x = 20\,\text{mm}$, $y = 20\,\text{mm}$, $P = 780\,\text{mm}$ of mercury
- B$x = 20\,\text{mm}$, $y = 30\,\text{mm}$, $P = 780\,\text{mm}$ of mercury
- C$x = 30\,\text{mm}$, $y = 20\,\text{mm}$, $P = 810\,\text{mm}$ of mercury
- D$x = 30\,\text{mm}$, $y = 30\,\text{mm}$, $P = 790\,\text{mm}$ of mercury