Why are two holes made to empty an oil tin?

From the adjacent figure,the correct observation is:

The density of the atmosphere is $1.29 \ kg/m^3$. If the atmospheric pressure is $1.013 \times 10^5 \ Pa$,how high would the atmosphere extend if the density were uniform? $(g = 9.81 \ m/s^2)$

$A$ pressure of $1 \,mm$ of mercury is equivalent to

What is the pressure in $atm$ on a swimmer $10\; m$ below the surface of a lake?

$A$ tank of oil has a height of $4 \,m$ and a density of $850 \,kg \,m^{-3}$. The gauge pressure at the bottom of the tank is (Given: $1 \,atm = 10^5 \,Pa$,Acceleration due to gravity $g = 10 \,m \,s^{-2}$) (in $\,kPa$)