Estimate the Observed Wavelength for The $656 \mathrm {~nm}$ Line in the Spectrum of a Star Which Is 100

Multiple Choice

Estimate the observed wavelength for the $656 \mathrm {~nm}$ line in the spectrum of a star which is 100 million light-years from us if $H = 20 \frac { \mathrm { km } / \mathrm { s } } { \mathrm { Mly } }$ ? $\left( c = 3.00 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } \right)$

A) $660 \mathrm {~nm}$
B) $640 \mathrm {~nm}$
C) $690 \mathrm {~nm}$
D) $620 \mathrm {~nm}$