How is modern Earth different from Earth over four billion years ago?

a) Constantly bombarded with comets and asteroids

b) Little to no oxygen

c) High levels of ultraviolet radiation

d) Protective atmosphere

B. Little to no oxygen

Explanation:

The atmosphere on the primitive Earth over four billion years ago was reducing. It did not have oxygen or oxygen was present in very little amount. The present-day atmosphere of today's Earth is rich in oxygen.

Evolution of oxygen-evolving autotrophic cyanobacteria led to the accumulation of oxygen in the atmosphere of primitive Earth.

D, over the past Billions of years, our atmosphere has formed to protect life.