Why Will Proposed Future Experiments to Search for Earthlike Planets

Why will proposed future experiments to search for Earthlike planets use the infrared region of the spectrum?

A) Life forms give off IR radiation preferentially when they are alive (and warm!) .
B) The ratio of IR to visible radiation from planets is much higher than for their parent stars.
C) Earth's atmosphere is more transparent over this wavelength range than at any other wavelength.
D) Only at these wavelengths will spectra show spectral signatures that are identifiable as originating on Earthlike planets.