Which explanation is true for the mechanical pressure (pm) and thermodynamic pressure (pth) ?

• A. A pressure sensor records pth. However, for most of the practical engineering cases, the system at the intermediate states of a process gets enough opportunity to attain equilibrium. Thus, both pth and pm are equivalent in these cases.
• B. pth is the average mechanical pressure exerted at a point in the fluid but pm is defined as the force exerted on container walls as fluid molecules coincide with it during their random movement.
• C. A pm is a variable quantity and it depends upon the coordinate system used.
• D. A pressure sensor records pth. However, for most of the practical engineering cases, the system at the intermediate states of a process does not get enough opportunity to attain equilibrium. Thus, both pth and pm are equivalent in these cases.