Why Wasn't the Intracluster Medium in Galaxy Clusters Discovered Until

Why wasn't the intracluster medium in galaxy clusters discovered until the 1960s?

A) We did not know how much dark matter existed before then.
B) We didn't have the resolution to observe galaxy clusters until then.
C) The Milky Way was blocking our view of distant galaxy clusters.
D) The medium emits X-rays,which are blocked by the Earth's atmosphere and require X-ray satellites in space in order to be observed.
E) Radiation emitted by the medium was so dim that we couldn't detect it until we built much larger telescopes.