Which gas was not present around the time earth first formed?

methane
hydrogen
oxygen
nitrogen

Oxygen was not present around the time earth first formed. Hope this helps. :)

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms