Fluid Mechanics
The variable required to be known in correlations used for estimating the horse power of a centrifugal gas compressor and hence its cost are P. Inlet pressure Q. Compressor rpm R. Delivery pressure S. Volumetric flow rate at inlet.

P, Rand S
P, Q and R
P and R
R and S

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Fluid Mechanics
A stream line is

Always the path of particle
A line connecting the mid points of flow cross-section
Drawn normal to the velocity vector at every point
Fixed in space in steady flow

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