$$Part $($a$)$

Three students are debating:

Student A: The pressure at level C must now be greater than the atmospheric pressure because liquid there is being pushed up against the stopper.

Student B: I think the pressure at level B must be the same as at level A$,$ because they are at the same level. Both are at atmospheric pressure. So the pressure at level C must be lower than atmospheric, because pressure decreases as you ascend.

Student C: But the liquid is denser than air, so the pressure at C cannot be less than atmospheric pressure.