Choose the sentence in this excerpt that explains why the cosmic microwave background experienced redshift.
When the residual radiation was first emitted just a few hundred thousand years after the Big Bang, it was observable at a short ultraviolet wavelength. As the universe expanded, however, the ultraviolet radiation experienced a phenomenon called redshift, whereby the wavelengths of the photons stretched to such a large degree that they transitioned into the low-energy red domain on the electromagnetic spectrum. In other words, redshift represents a loss of energy within photons as they transition from the high-energy blue end of the electromagnetic spectrum into the red domain of the spectrum. Today, after billions of years of cosmic inflation, the CMB is observable only
A. at a low-energy red wavelength.
B. at a high-energy blue wavelength.
C. in the ultraviolet range.
D. at the same wavelength as originally emitted.