Using the principle of faunal succession, if an unidentified fossil is found in the same rock layer as an index fossil, the two species must have existed during the same period of time (Figure 4). If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time.
I think the answer is C.
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What they mean by "base" is the substance. A base is a substance that tastes bitter, has a pH that is greater than 7. It also turns red litmus paper blue.
In conclusion, what they mean by 'base' is the substance included.
