What electron transition in a hydrogen atom,starting from the orbit $n=7$ will produce infrared light of wavelength 2170nm?( $R_H=1.09677\times 10^7m^{-1})$

Question

$\begin{array}{1 1}(a)\;n=2\;to\;n=3&(b)\;n=6\;to\;n=7\\(c)\;n=7\;to\;n=4&(d)\;n=4\;to\;n=7\end{array}$