(need fast)which unit of geologic time is the oldest?
a)cenozoic
b)mesozoic
c)ordovician
d)precambrian

D) Precambrian

Explanation:

Option D, Precambrian​

Explanation:

Cenozoic period existed some 65 million years ago when the largest animal species evolved on planet earth. Mesozoic is a period that existed just before the Cenozoic period and lasted from 252 to 65 million year ago. Ordovician period started some 485 million years ago and existed between the Cambrian and Silurian period. Pre Cambrian period occurred some 4600 million years ago.  

Hence, Pre Cambrian period is the oldest geological time  

Option D is correct.

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explanation:

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succession returns a disturbed

ecosystem to a more stable state. this is also classified into two (2) types:

primary succession and secondary succession. the primary succession arises when

a new land is transformed and a bare rock has been exposed providing an environment

that can be occupied for the first time. while, secondary succession occurs

when a previously occupied environment was disturbed that kills much of all its

community

explanation: