As Astronomers Look at More Distant Regions of Space, They

Multiple Choice

As astronomers look at more distant regions of space, they see the regions as they existed at earlier times, but their farthest views are blocked by a "wall" beyond which the universe is opaque. What event occurred at the time marked by this wall?

A) Quarks combined to form neutrons and protons.
B) Electrons and protons combined to form neutral hydrogen atoms.
C) Protons combined with neutrons to form helium nuclei.
D) Gravity froze out as a separate force.