An Astronomer Wants to Observe a Cloud of Cold Neutral

Multiple Choice

An astronomer wants to observe a cloud of cold neutral (not ionized) hydrogen, far away from any stars. What would be an instrument that could help in this task?

A) a telescope in space, able to pick up x-rays from space
B) a radio telescope, tuned to a wavelength of about 21 centimeters
C) a large telescope observing visible light, but with a very sensitive CCD detector attached
D) a neutrino detector, deep underground
E) you can't fool me; there is no way to observe clouds of hydrogen that are not near a star