Two Observers Are on Different Floors of a Building, as Shown

Multiple Choice

Two observers are on different floors of a building, as shown in the figure below. The observer on the bottom floor shines a light upward, which is reflected from a mirror in an upper floor and sent back downward. Consider the gravitational redshift of the light due to the Earth. If the light has a wavelength of exactly 600 nm when emitted, the observer on the upper floor will measure a wavelength of __________ while the observer on the lower floor will see the light returning with a wavelength of __________.

A) "600 nm, 600 nm"
B) " $\gt$ 600 nm, 600 nm"
C) " $\lt$ 600 nm, $\lt$ 600 nm"
D) "600 nm, $\gt$ 600 nm"
E) " $\gt$ 600 nm, $\gt$ 600 nm"