Hydraulics & Fluid Mechanics
Mercury is often used in barometer because

it is the best liquid
both (B) and (C)
the height of barometer will be less
its vapour pressure is so low that it may be neglected

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Hydraulics & Fluid Mechanics
The normal stress is same in all directions at a point in a fluid

When there is no motion of one fluid layer relative to an adjacent layer
only when the fluid is incompressible and has zero viscosity
irrespective of the motion of one fluid layer relative to an adjacent layer
only when the fluid is frictionless

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