Decide whether the following statement makes sense (or is clearly true) or does not make sense (or is clearly false), and why.
In the distant past, the cosmic microwave background consisted primarily of infrared light.
A) The statement is true. The cosmic background was released when the universe had the temperature of about 3000 K. That is similar to the temperature of a red star, which emits mostly in infrared wavelengths. Then that wavelength was redshifted.
B) The statement is false. The cosmic background was released when the universe had the temperature of about 30 K. That is similar to the temperature of a molecular gas cloud, which emits mostly in radio wavelengths. Then that wavelength was only redshifted.
C) This statement does not make sense. The cosmic background was released when the universe had the temperature of about 15,000 K. That is similar to the temperature of the blue star, which has peak at about 200 nm, in the ultraviolet wavelength. Then that wavelength was only redshifted.
D) This statement makes sense. The cosmic background was released when the universe had the temperature of about 6000 K. That is similar to the temperature of the Sun, which has peak at about 500 nm, in the visible wavelength. Then that wavelength was redshifted.