Astronomers Can Now Report That Active Star Formation Was Going

Multiple Choice

Astronomers can now report that active star formation was going on at a time when the universe was only 20% as old as it is today. When astronomers make such a statement, how can they know what was happening inside galaxies way back then?

A) they look at the open clusters in the Milky Way Galaxy
B) they look at radio waves coming from nearby active galaxies
C) they look at a graph that shows the way galaxy redshifts change with time
D) they look at the elements in comets, which formed when our solar system was first forming
E) they examine the spectra of galaxies (or the overall colors of galaxies) with the highest redshifts they can find