Which of the following statements about a black body are true? Select one or more: a. The spectrum of the cosmic background radiation corresponds with great accuracy to the radiation of a black body at a temperature of 2.7 K. b. A black body absorbs all the radiation that hits it, and emits no radiation at all. C. According to Planck's radiation law (black body distribution), the wavelength corresponding to the maximum energy density of the radiation decreases (and the frequency increases) as the temperature increases. d. A black body reflects all the radiation that hits it, and absorbs no radiation at all.

Which of the following statements about a black body are true? Select one or more: a. The spectrum of the cosmic background radiation corresponds with great accuracy to the radiation of a black body at a temperature of 2.7 K. b. A black body absorbs all the radiation that hits it, and emits no radiation at all. C. According to Planck's radiation law (black body distribution), the wavelength corresponding to the maximum energy density of the radiation decreases (and the frequency increases) as the temperature increases. d. A black body reflects all the radiation that hits it, and absorbs no radiation at all.