Many of the Telescopes Sent into Space Have to Be

Many of the telescopes sent into space have to be cooled to liquid helium temperatures. Why is this?

A) The telescope will expand to a distorted shape if it is not cooled to the temperature of its surroundings. This cooling would not be necessary if the telescope were on Earth.
B) The telescope will radiate as a blackbody, and this radiation will obscure the astronomical signal. This blackbody radiation is reduced at cooler temperatures.
C) The cooler temperatures reduce the wavelengths of the incoming radiation, thus increasing the resolution.
D) The liquid helium is vaporized and used to produce a reference spectrum for comparison with the spectra of the astronomical sources.