In the pure rotational microwave spectrum of a XY molecule, the adjacent lines are separated by $4\,\mathrm{cm^{-1}}$. If the molecule is irradiated by a radiation of $30{,}000\,\mathrm{cm^{-1}}$, the first Stokes line (in $\mathrm{cm^{-1}}$) appears at
(a)29988
(b)30012
(c)30004
(d)29996
Answer
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