(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:

hydroxyl -oh

alcohols, always polar, can form hydrogen bonds with h2o molecules

carboxyl -cooh  

cargoxylic acids, has acidic properties which is a source of hydrogen ions, found in cells in ionized for with a charge of oh-1

carbonyl > co

ketone if within skeleton, aldehyde if group is at the end of skeleton, k & a may be structural isomers, it determines 2 sugars (ketose & aldose)

amino -nh2

amines, acts as a base, it can pick up h+ ions from surrounding solution, ionized with charge of 1+

methyl ch2

addition to rna/dna affects and gene expression

phosphate -opo3 ²⁻

organic phosphates, contributes to negative charge to, molecule that it is a part of, and potential to react with h20

sulfhydryl -sh

thiols, cross-linking stabilize the protein, and it is structure