Penzias and Wilson Had No Difficulty Making Their Initial Observation

Penzias and Wilson had no difficulty making their initial observation of the cosmic microwave background radiation at a wavelength of 7.35 millimeters.But it required other telescopes to verify the complete blackbody spectrum of the radiation.Why? (See Figure 14-4,Comins,Discovering the Essential Universe,6th ed.)

A) The microwave receiver of Penzias and Wilson could be pointed at only one region of the sky, and a complete sky scan was necessary for the full blackbody curve.
B) Penzias and Wilson had, by coincidence, been observing the curve near its peak, where the intensity was strongest. The rest of the curve required more sensitive receivers.
C) The peak of the curve and the sharp fall-off on the short wavelength side occur at wavelengths of 1 millimeter or shorter, and this radiation is blocked by Earth's atmosphere. Thus, balloons and satellites had to be used.
D) Because of the changing seasons, the source of this radiation moved out of the night sky for Penzias and Wilson soon after they made their initial observation, so further measurements had to be made by observers elsewhere on Earth.