Gas Chromatography
Helium is generally preferred as carrier gas over nitrogen and hydrogen because

it doubles up as a party gas for balloons and funny voices
it has a lower viscosity
it is inert
all of above

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Gas Chromatography
Theoretical plates are used to

determine the thickness of the stationary phase
estimate the efficiency of a column
None of these
measure the distribution of the analyte between mobile and stationary phases

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