When the mercury column does not enter the thin tube, the pressure of the enclosed gas is constant and changes at constant pressure. According to Gay-Lussac's law, volume is proportional to thermodynamic temperature, and V-t is like an inclined straight line passing through the origin.

When the mercury column enters the thin tube, the pressure of the enclosed gas gradually decreases. It can be seen from the question that t increases and v increases.

VT=nRP can be obtained from the gas equation PVT=nR,

The slope of the straight line between the point on the graph and the origin is K=VT, and when p decreases, k increases.

When the mercury column completely enters the thick pipe, the pressure of the enclosed gas remains constant and changes at constant pressure. According to Guy Lussac's law, volume is directly proportional to thermodynamic temperature, and vt = c'= nrp' ′. The V-t image is also an inclined straight line passing through the origin. Because p > p', the slope of this straight line increases.

So choose a.