[填空题]
Estimate the observed wavelength fmz 5mm.(ybn:ixz 8s4oor the 656-nm line in the Balmer series of7 gxlu0s;g/ icwh8ut. hydrogen w xu.8 gh7;g/lts0 iucin the spectrum of a galaxy whose distance from us is
(a) $1.0 \times 10^6\ \text{ly}$, $\approx$ nm
(b) $1.0 \times 10^8\ \text{ly}$, $\approx$ nm
(c) $1.0 \times 10^{10}\ \text{ly}$. $\approx$ nm
