Textbook Problem

widely Used weather instrument called a barometer can be built from a long, thin tube of glass that is sealed at one end. The tube is completely filled with mercury and then inverted into a small pool 0f mercury. The level of the mercury inside the tube drops initially but then stabilizes at some height. A measure of the height of the column of mercury once it stabilizes is a measure of pressure in mm Hg (or torr). Which of the following is the best explanation of how this barometer works?

Air pressure outside the tube (pressure of the atmosphere) counterbalances the weight of the mercury inside the tube.

Air pressure inside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal.

Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal.

The vacuum that is formed at the top of the tube of mercury (once the mercury level in the tube has dropped same) holds up the mercury.

I have no idea how a barometer works.