Why Would Intelligent Alien Beings Wanting to Communicate with Us

Multiple Choice

Why would intelligent alien beings wanting to communicate with us probably choose the 21-cm atomic hydrogen radio wavelength?

A) This wavelength shows a very strong Doppler effect when its source is moving, and intelligent alien beings would know that we would be able to detect the orbital motion of their home planet around their star by this method.
B) Intelligent alien beings would expect that many of our telescopes would already be tuned to this precise wavelength for scientific work.
C) This radiation at this particular wavelength is very weak from natural sources in space, and messages would be easily distinguished from other sources.
D) Intelligent alien beings would have detected this particular wavelength from our transmitters on Earth since it is used extensively for satellite communications, and they would know that we could detect them easily.