A tube sealed at both ends is 100cm long.It lies horizontally,the middle 0.1m containing mercury & 2 ends containing air at standard atmospheric pressure.If tube is turned to vertical position,what amount of mercury will be displaced?

Question

$\begin{array}{1 1}(A)\;1cm\\(B)\;2cm\\(C)\;3cm\\(D)\;4cm\end{array}$